As lots of people have said it is a very individual thing. Noone can tell you what will suit you or your family but you and your partner.
I have 16mths between my first two, and while it is great now, when they were smaller it was pretty difficult for me to cope. They both didn't sleep well, DS was a surprise little gift and whist we were planning on always having 2-3 kids, he came a fair bit earlier than we had expected. DS was full breastfed and was a booby monster and DD wanted me to be with her all the time too, only being a baby herself. I had wanted to wait til DD was nearing 2, so having a 2 1/2-3yr gap between kids.
But in saying that DD and DS are best buddies now and all of the really 'baby' sort of stuff is over. With this bub, he will be almost 3 and 2 1/2 years apart from his siblings. I think this gap will be alot easier in terms of being able to do things for bub without the other two being in the way or being so young they need the same sort of care, itms? For example, DD is a pretty independant little person. She dresses herself, feeds herself, often sleeps through the night, is toilet trained, can get things for herself (within reason!) and understands alot, and helps me a fair bit with tidying and stuff like that. DS will hopefully be toilet trained soon (possibly even before bub is here) , can mostly feed himself, he's getting better with sleeping and will hopefully be better still with sleeping when bub is born, he is no longer bfing, can understand a little and helps a little so in 3 months time, he will have more understanding again. They also play together and entertain each other so I will hopefully be able to focus on bub, with things like feeding, bathing ect...
So IMO, I think 2-3yrs is a good age for siblings to come along! Kids are more self sufficient at that age and can understand when spoken to, they're willing to help and generally do what you ask (usually!!).