I am very sorry for the diagnosis, but you made a very smart move getting that amputation done a.s.a.p before any sign of metastasis. In my experience, in which we've dealt with this many many times, cats do amazing well with 3 legs. In fact, the majority of cats we've amputated don't seem to even notice a difference and are moving about upon waking up from anesthesia. It's more difficult in dogs with broad chests and narrow waists to distribute their weight differently, but cats, being much closer to the ground already and having amazing sense of balance, rarely have a problem. A couple I have seen avoided moving much about 2 days post-op, but that was more likely to being a bit out of it from the amount of pain meds on board, and started slithering soon after, then walking about with no problems. I've never seen a cat not able to move about ever after an amputation, so don't worry about that.
While you'll likely have some future metastasis checks (chest x-rays, maybe an ultrasound far in the future), having had this done gave your cat an amazing chance at a well-extended life now, and I'm smiling to hear how quickly you acted on this and your cat's chances, which are extremely good.
You should feel very good about this now and I'm sure your cat will be recovering quickly from this.
Again, good luck with everything, and I'm still available on this thread should you need anything further.