Firstly congratulations on No. 3. My circumstances are a bit different in that it is my 3rd child that has special needs. Saying that, if she had been first-born (or second), I like to think I would have gone on to have No. 2 and/or 3.
Also, my daughter was 9 when the youngest was born and my son was 4.5 (they are now 14, 9 (nearly 10) and 5 (SN), although her special needs did not appear until she was 9 months or so (although I knew something was not quite right from an earlier age).
The one thing I found hardest (getting easier now that some of the therapists are not so involved now that she is at school) was making sure that, as a single parent (husband left when youngest was 11 weeks old) I gave my elder two the time and attention they deserve.
Now I do still find that I spend more time with my youngest (hospital appointments, etc.) but that time is not "quality" time, it is "necessity" time and I try to give each of them the time and attention they each need. I don't want them feeling resentful towards the youngest because of her need (although they do seem to be very understanding and accepting of her).
Me and my eldest now like to have "girly" nights (watching a girly film) after the others are in bed. The film may not be my choice but it gives me time with my DD on our own.
My son loves playing board games (as well as being a typical boy and liking his PS2, PSP, etc.) but I try and make sure that once the youngest is in bed, at the weekend, on a Saturday night, we play a board game together. I also sit with him every night and, although he is a very fluent reader (reading age 14), I like him to read to me as it gives us time together.
Although I spend lots of time with youngest DD with appointments, I also try to spend time with her doing "fun" things, making cakes, playing teacher, doing puzzles (one of her abilities).
I would say my biggest problem is the age gap and the genders involved (when DD2 was born, DD1 was 9). That means that when DD2 is 9 her sister will be 18 (scary thought).
Although DD has SN, I would not be without her, being the way she is. It may sound strange, but through the marriage break-down she kept me sane because I was so caught up in everything that was happening to her I did not have time to feel sorry for myself or let myself get down. I just had to deal with it.
One of the "nicest" things my eldest said, when DD2 was first diagnosed, was that she was 'glad' that she had Sotos and wouldn't want to change her because having Sotos has made her the person she is, and if she didn't have it, then she wouldn't be who she was (and is now).
Hope that helps and congratulations.