I was the oldest of 3 and then at 15 through re marriages, suddenly became the oldest of 6!
I liked being both, actually (though don't aspire to a family of 6!). Me and my sister have never really managed to get on, but I'm close to my little brother.
I'm also a mum of three. I'd somewhat reluctantly decided to stop at 2, which was what dh wanted and pregnancy and me really don't get on at all well, I was in and out of hospital for months. I had a wee regret but thought it was for the best when my mirena coil became dislodged after a smear and I found myself unwittingly almost through the first trimester!
It was an awful pregnancy too, but dd2 beautifully completes our family and we'd not be without her. I did have 3 under 4 though - when dd2 was born, dd1 was 3y10m and ds was 1y10m. Very hard work! But the jump of 2 -> 3 was really easy compared to 1 -> 2 which I found ghastly. Dd2 was a very easy baby (though she's high maintenance now!) and just slotted right in. I too was worried about getting horribly sleep deprived, and I did get PND again, but like you, my husband is a doctor (he's a GP) and was on a 1in3 on call rota at the time in a very rural area. He did help loads at the weekends and at least o/c was from home, but it added to the strain.
Everyone is shouty stressed mum on occasion, though. I was f/t at home (did a bit of freelance work when I felt the need) and it took me almost 2 years to fully recover from the pregnancy. I had a mother's help twice a week who would tidy, iron, clean, entertain the kids a bit, pop in for some shopping etc. I and the kids adored her, when we moved away they were distraught, poor dd2 thought that when we said we were all moving, Isabel was included!!
I think it's good to consider age - I actually had an early menopause before I turned 40 so if I'd waited things would have been very different.
Hope you can make a decision you are happy with (for all of you) and sorry for the essay 