Orm, have definitely felt like this at times. Have a 9 year old DS and 6 year old DTD's and often remember when it was 'just the two of us'...just me and my boy.
I absolutely love my DTD's, would not and could not be without them but it doesn't stop me thinking how things would be if I only had the one.
Having 3 is hard...especially as I'm a single parent.
The one thing that pulls me out of it is this. One of the reasons I wanted a sibling for my DS is because I am an only child myself. Although it didn't bother me much as a kid, I found that into my teens, twenties, now even, I so wish I had a brother or sister, someone who 'shares' my history, someone I can reminisce(sp?) with about the past, someone who 'gets me' and has been where I have been, someone who can validate things.
It is the future I think about, the fact they will all have each other, hopefully they will lean on each other in times of troubles etc, there will be no pressure on just the one to deal with it all, like there was with me.
I also think of all the aunts, uncles, cousins, grandchildren etc that will be created by my having 3 children....and it gives me a warm glow .
HTH