The book is Science Fiction Terror Tales, a 1955 anthology edited by Groff Conklin. It has appeared under several covers; the one with the red cover is the 1969 Pocket Books edition:
I remember 3 stories. One about a blob that lands on Earth and absorbs matter exponentially. They launch it into space and blow it up, and now infinite pieces will land on more planets.
"The Leech", a short story by Robert Sheckley, available at Project Gutenberg and the Internet Archive.
The leech was waiting for food. For millennia it had been drifting across the vast emptiness of space. Without consciousness, it had spent the countless centuries in the void between the stars. It was unaware when it finally reached a sun. Life-giving radiation flared around the hard, dry spore. Gravitation tugged at it.
A planet claimed it, with other stellar debris, and the leech fell, still dead-seeming within its tough spore case.
One speck of dust among many, the winds blew it around the Earth, played with it, and let it fall.
On the ground, it began to stir. Nourishment soaked in, permeating the spore case. It grew—and fed.
They launch it into space and blow it up, and now infinite pieces will land on more planets.
It had been shrinking from the expenditure of energy, when the great explosion came. No thought of containing it. The leech's cells held for the barest fraction of a second, and then spontaneously overloaded.
The leech was smashed, broken up, destroyed. It was split into a thousand particles, and the particles were split a million times more.
The particles were thrown out on the wave front of the explosion, and they split further, spontaneously.
Into spores.
The spores closed into dry, hard, seemingly lifeless specks of dust, billions of them, scattered, drifting. Unconscious, they floated in the emptiness of space.
Billions of them, waiting to be fed.
The second is about a human-alien war where time freezes and there's a 1v1 duel in a sand pit with a barrier in the center. The alien is an armadillo type creature.
"Arena", a novelette by Fredric Brown; also the answer to several old questions, such as Human & Alien captured, observed by higher beings; first published in Astounding Science Fiction, June 1944, available at the Internet Archive. A very famous story.
From the Wikipedia plot summary:
The mysterious Outsiders have skirmished with Earth's space colonies and starships. Their vessels are found to be faster and more maneuverable, but less well armed. There have been no survivors of the small raids on Earth forces so Earth has no information about the Outsiders. Fearing the worst, Earth builds a war fleet. Scouts report a large armada approaching the solar system. Earth's defenders go to meet them. All indications are that the two fleets are evenly matched.
Bob Carson is the pilot of a small one-man scout ship on the outskirts of the fleet. While engaging his Outsider counterpart in battle, he blacks out. When he awakens, he finds himself naked in a small enclosed, circular area about 250 yards (230 m) across. Other than vegetation and blue sand, he sees in the distance only a red sphere about 1 yard (0.91 m) in diameter. The sphere turns out to be an Outsider, with several dozen fully retractable thin tentacles to manipulate objects. Based on its method of movement, Carson labels it a "Roller".
The third is about a planet of robots who have no idea what humans are, finding a space shuttle and thinking it's another robot.
"Lost Memory", a short story by Peter Phillips, also the answer to the old question Robots on planet with no humans until a rocket with a human lands; first published in Galaxy Science Fiction, May 1952, available at the Internet Archive.
ISFDB synopsis:
A spaceship crashes on a planet of intelligent machines who attempt to come to its aid, but are unable to grasp the concept that the intelligence piloting it may not be mechanical.
They talk to it and then try to fix the damage, killing the man in the process by burning him alive.
The story is told by a robot reporter present at the "rescue" operation:
Rivulets of metal ran down into a tray which a calm assistant had placed on the ground for that purpose. I averted my eyes quickly. I could never steel myself enough to be a surgical engineer or assembly technician.
But I had to look again, fascinated. The whole area circumscribed by the marking was beginning to glow.
Abruptly the stranger's voice returned, quite strongly, each word clipped, emphasized, high-pitched.
"Ar no no no . . . god my hands . . . they're burning through the lock and I can't get back I can't get away . . . stop it you feens stop it can't you hear . . . I'll be burned to deth I'm here in the airlock . . . the air's getting hot you're burning me alive . . ."
Although the words made little sense, I could guess what had happened and I was horrified.
"Stop, Chur-chur," I pleaded. "The heat has somehow brought back his skin currents. It's hurting him."
Chur-chur said reassuringly: "Sorry, Palil. It occasionally happens during an operation—probably a local thermo-electric effect. But even if his contact senses have started working again and he can't switch them off, he won't have to bear this very long."
