I am looking for a book. Written before 1975. It is about earth being attacked by an alien force. Earth builds a ship loaded with earth's finest. As they travel to a distant world they are attacked and land on a planet. Generations go by on this world that has extreme gravity and they develop. Talking mind reading penguins and vicious wolves. They send a signal back to earth "Raroak Calling"... They eventually defeat an alien ship and return to earth. They defeat the alien attack force because after generations on the alien planet they developed super human strength because of the 3x gravity. Then they must leave earth because they realize they are no longer human.
The message is "Ragnarok calling". The book is Tom Godwin's 1958 novel The Survivors a.k.a.Space Prison; it's available at Project Gutenberg and LibriVox. Maybe one of these covers will ring a bell. A shorter version called "Too Soon to Die" was published as a novelette in Venture Science Fiction Magazine, March 1957, which is available at the Internet Archive. There was a sequel called The Space Barbarians. Godwin's Ragnarok series (as it's called) was the subject of this old question and this one and this one. From the Wikipedia summary:
A ship heading from Earth to Athena, a planet 500 light years away, is suddenly attacked by the Gerns, an alien empire in its expansion phase. People aboard are divided by the invaders into Acceptables and Rejects. The Acceptables would become slave labor for the Gerns on Athena, and the Rejects are forced ashore on the nearest 'Earth-like' planet, called Ragnarok. The Gerns say they will return for the Rejects, but the Rejects quickly realise that that isn't going to happen.
Ragnarok is not so 'Earth-like.' Its gravity is 1.5 times that of Earth, it is populated by deadly, aggressive creatures and it contains little in the way of usable metal ores. This, combined with a terrible deadly fever that kills in hours, more than decimates the population.
The novels follows the stranded humans through several generations as they try to survive there, and their unswerving goal to repay the Gerns for their cruelty.
Now to address some points in your question with excerpts from the story:
Talking mind reading penguins
The mind-reading critters, called "mockers", are more like squirrels:
They emerged into full view; six little animals the size of squirrels, each of them a different color. They walked on short hind legs like miniature bears and the dark eyes in the bear-chipmunk faces were fixed on him with intense interest.
[. . . .]
"They're telepathic between one another," he said. "The yellow one there repeated what the one you spoke to heard you say and it repeated what the yellow one heard me say. It has to be telepathy between them."
[. . . .]
Their intelligence was surprising and they seemed to be partially receptive to human thoughts, as Bill Humbolt had written. By the end of the fifteenth year their training had reached such a stage of perfection that a mocker would transmit or not transmit with only the unspoken thought of its master to tell it which it should be. In addition, they would transmit the message to whichever mocker their master's thought directed. Presumably all mockers received the message but only the mocker to whom it was addressed would repeat it aloud.
and vicious wolves.
The warning cry came from an outer guard and black shadows were suddenly sweeping out of the dark dawn.
They were things that might have been half wolf, half tiger; each of them three hundred pounds of incredible ferocity with eyes blazing like yellow fire in their white-fanged tiger-wolf faces. They came like the wind, in a flowing black wave, and ripped through the outer guard line as though it had not existed. The inner guards fired in a chattering roll of gunshots, trying to turn them, and Prentiss's rifle licked out pale tongues of flame as he added his own fire. The prowlers came on, breaking through, but part of them went down and the others were swerved by the fire so that they struck only the outer edge of the area where the Rejects were grouped.
At that distance they blended into the dark ground so that he could not find them in the sights of his rifle. He could only watch helplessly and see a dark-haired woman caught in their path, trying to run with a child in her arms and already knowing it was too late. A man was running toward her, slow in the high gravity, an axe in his hands and his cursing a raging, savage snarl. For a moment her white face was turned in helpless appeal to him and the others; then the prowlers were upon her and she fell, deliberately, going to the ground with her child hugged in her arms beneath her so that her body would protect it.
The prowlers passed over her, pausing for an instant to slash the life from her, and raced on again. The vanished back into the outer darkness, the farther guards firing futilely, and there was a silence but for the distant, hysterical sobbing of a woman.
It had happened within seconds; the fifth prowler attach that night and the mildest.
They send a signal back to Earth "Raroak Calling"...
Not to Earth but to the nearest enemy aliens, believed to be on Athena, 200 light-years from Ragnarok:
It was five years before the transmitter was ready for testing. It was early fall of the year thirty-five then, and the water that gushed from the pipe splashed in cold drops against Humbolt as the waterwheel was set in motion.
The generator began to hum and George observed the output of it and the transmitter as registered by the various meters he had made.
"Weak, but it will reach the Gern monitor station on Athena," he said, "It's ready to send—what do you want to say?"
"Make it something short," he said. "Make it, 'Ragnarok calling.'"
George poised his finger over the transmitting key. "This will set forces in motion that can never be recalled. What we do here this morning is going to cause a lot of Gerns—or Ragnarok people—to die."
"It will be the Gerns who die," he said. "Send the signal."
"Like you, I believe the same thing," George said. "I have to believe it because that's the way I want it to be. I hope you're right. It's something we'll never know."
He began depressing the key.
They eventually defeat an alien ship and return to Earth. They defeat the alien attack force because after generations on the alien planet they developed super human strength because of the 3x gravity.
Not just the higher gravity (1.5 g, not 3 g) but the generally harsh conditions on Ragnarok have made the survivors physically, mentally, and morally stronger. That, and their alliances with the initially hostile natives/wildlife, enable them to defeat the Gerns.
"No one expected you to survive here." The commander wiped at his swollen lips, wincing, and an almost child-like petulance came into his tone. "You weren't supposed to survive."
"I know," he said again. "We've made it a point to remember that."
"The gravity, the heat and cold and fever, the animals—why didn't they kill you?"
"They tried," he said. "But we fought back. And we had a goal—to meet you Gerns again. You left us on a world that had no resources. Only enemies who would kill us—the gravity, the prowlers, the unicorns. So we made them our resources. We adapted to the gravity that was supposed to kill us and became stronger and quicker than Gerns. We made allies of the prowlers and unicorns who were supposed to be our executioners and used them tonight to help us kill Gerns. So now we have your ship.
Then they must leave Earth because they realize they are no longer human.
They don't actually get to Earth in Space Prison/The Survivors; I suppose that happens in the sequel, which I haven't read. But they anticipate the problem:
"One day more," Craig said. "We're two hundred years late but we're coming in to the world that was to have been our home."
"It can never be, now," he said. "Have any of us ever thought of that—that we're different to humans and there's no human world we could ever call home?"
"I've thought of it," Lake said. "Ragnarok made us different physically and different in the way we think.We could live on human worlds—but we would always be a race apart and never really belong there."