I know its not exactly what you asked, as I don't have a story of regretting not having a third child but I thought you might like to hear my story, as I was in your position eighteen months ago. I was 38 and had two dss aged 4 and 6. We hivered and hovered back and forth between going for another or stopping at two, we weren't in the best financial situation but felt that time was marching on and we couldn't leave the decision any longer.
In the end we had a really long, completely honest talk about whether we just liked the idea of having another 'baby' or we actually wanted another 'child'. (To begin with it was me that wanted another and dh was totally anti.) Once we talked it through properly though, we were happy that we both felt we would like another child felt we would both regret it if we didn't go ahead. So, we went for it and I now have a beautiful 9 month old dd.
She fitted right in straight away, her brothers absolutely worship her and there has been no jealousy whatsoever (unlike when ds2 was born and ds1 was 2 years old).
For us it has been a really positive experience. Dd is an absolute treasure. I found going from 2 to 3 easier than 1 to 2 (probably as the boys are at school all day so I have time to spend with dd when they are out and then don't have to feel guilty if she has to wait while I spend time with them after school).
When I thought about having a third child, I imagined in my head our little family of four as a sort of square shape, when I imagined adding another child into the mix the square sort of morphed into a circle and felt much cosier and more like a hug. [daft old hippie emoticon]
I know that sounds daft, but it has been a bit like that for us. Before it tended to be dh and ds1 and me and ds2, but now we seem somehow to be more of a family and there aren't two separate factions these days, we all muck in together.
I'm not saying its all been plain sailing. The pregnancy was hard as I was quite poorly for most of it. (Nothing major, just nasty morning sickness, anaemia and total all encompassing exhaustion). It can be tiring having three dcs who all want/need things 24/7 at times and we do get tired, but all things considered, we are both very happy with our decision and wouldn't change things for the world.
I wish you all the best with your decision making. Hope it all works out for you, whatever you decide.