The solution to getting out with three is completely different to start from what you did to get out with one starting with one. And yet, in some ways it's actually much simpler. The cats you will leave the room with are one of your starting cats, and both of the revived cats.
You sacrifice one of your starting cats very early in the level to make it possible to get the other cats past the second medusa. The movements you have to make are very tight though, because you don't have a lot of free space to move around without leaving the room, stoning the wrong cat, or jamming the dead cat into the wall by mistake.
And if it still seems impossible, remember the cat gets stoned at end of turn, so you might just think you can't push a medusa the right way.