Only you can decide really - Ds1 and 2 both have a disability but not the same one. They are not disabilities which you can screen for but any other baby I had was at an inflated risk of having either or both. I really, really wanted a third child and spent 3 years thinking it all through.
It was a massive decision as I knew if a third child was affected, it could put an unbearable strain on an already overstretched family. My parents really didn't want me too have another and were quite vocal about it.
In the end, I decided that I would cope if it came to it, DH and I have a strong marriage and we would pull together.
So I went for it, knowing if dc3 had a disability, especially ds2's, we would be very upset, but that we would cope.
In the end, our 3rd baby is 2 and developing normally. I'm so glad we did it, but I still see what we did as part informed choice, part gamble.
Good luck with your decision.