I'm not strictly in the same situation as you as I have one in between, but ds will be 5 and a half by the time no3 arrives.
He is so excited about baby; something he wasn't when I had dd because he was only 2. Now he's asking all sorts of questions and looking at baby books with me - he loves all the internal anatomy side of things and keeps quizzing me on umbilical cords [grins]
He's in primary one and so out of the house for most of the day. HOwever dd at 2.5 will be starting nursery after the summer and is pretty independent already. She's a lot more socially confident and self assured than ds was at this age.
So I think it really depends on the child.
I love the way my children interact with each other with an almost 3 year gap. They get on brilliantly. DD hasn't had my undivided attention as such, although she does from 8:30-4:40 every day now ds is at school. We didn't need a 6 year gap for that to happen.
I definitely couldn't do a teeny weeny age gap and have 2 or 3 under 2 or 3, but I'm happy with the gap we've got. I'll only have one in nappies for example
I think what I'm saying is, 6 years would be fine, but 3,4 or 5 would also work. Little girls love babies and love to feel like they're "helping". I'm looking forward to seeing my dd at 3 being big sis and I'm confident enough in her undersatnding at this age, that she will get that there will be times when she needs to share mummy.
Ds has struggled with thinking the world revolves around him and finds sharing hard, so having dd and soon dc3 will help him socially more than it will hinder him I think.
hth