All the tripods that come into our care seem to do very well indeed. We've had a thread on here recently where one of the posters had three three-legged cats I believe. I'll have to try to find it.
It could be that they would be at a disadvantage in escaping threats, such as dogs, so you may need to be careful where they have access to. However the RSPCA will go through all that with you and will assess whether or not you have a suitable environment for them. So as long as they are happy it shouldn't be a problem.
I had a three legged cat - she got run over and had to have her leg amputated, it was rear leg so easier for her - she was absolutely fine and lived to a ripe old age. The only thing is that she sometimes used to fall off the top of the sofa and would try and scratch herself with the missing leg.
I have two tripods and they are the best!! They are each missing a back leg (amputated after they had a bad time with a previous owner) which means they cannot jump up or climb so well, so I make sure they have platforms they can use to get up high in the house. This is important because all cats crave height. It also makes sure they can get on the bed etc. As for personalities, my boy is very anxious but my girl is a right scrapper and has taken on a huge fox before - and won! She also went through a phase of bringing in loads of birds, despite the missing leg and me putting a bell on her
The important thing to remember is that the cat won't know it's supposed to have four legs They will need you to give them a scratch sometimes and they will appreciate a good play at floor level. It's important to encourage them to stay active, and to watch their weight so they don't put strain on their remaining limbs, although most cats will regulate their own intake.
I hope you take a chance on a three-leg! I love mine to bits.