Sounds like "The Alien Dark."
The third planet is our Earth, long after humans managed to destroy all life on the planet. A bacteria intended to be used to pulp wood to make paper got out of control, and mutated. It ended destroying all plant life and making the Earth uninhabitable.
A group of scientists escape to a base on the moon and manage to stash away a bunch of frozen eggs and sperm of human beings. They tried to bring in a comet to provide water (and oxygen through hydrolysis,) but the plan fails.
Since they couldn't land the comet on the moon, the guys on the space ship managed to aim it at Venus. They got it to hit Venus at just the right angle to make it rotate at a comfortable speed and be better for supporting life.
By the time the cats show up on the scene, Venus has developed life (at least bacteria, maybe plants, I don't remember.)
The cats find the human base on the moon, and discover the "baby box" with everything it takes to fertilize the eggs and recreate people. They also find that there are a few frozen people from the last bunch that made it to the moon.
The story ends with the humans revived, and the threat of a second intelligent life form being enough to revive the cat race.
I liked the book, but the amazon reviews seem to give it a hard time.