I have a different, but strikingly similar situation.
DH and I have been together for 10 years. He had two DD with his ex, they are now 12 and nearly 18. They lived with mum, we had the girls weekends and shared school holidays. Worked brilliantly.
We had plans for one child, I only ever wanted one of my own, knowing that we had the girls too. Twins would have been the only way I'd have had more than one!
We then had the girls come to us full time totally out of the blue, when their mum (totally shockingly) was arrested and subsequently went to prison. I was pregnant at the time, and during the investigation/trial also lost my daughter, at 22 weeks. It's bloody horrific, and i honestly know how horrendous it is.
We have since had another happy, healthy baby, our daughter who is now 14 months.
I categorically only ever wanted one child. I now have three. I love them all to the ends of the earth, but is it what I imagined? Absolutely not. Is it the cards I've been dealt? yup. Only way I could change it would be to walk out on the girls that need me, with my daughter, separate her from her father and sisters who love her to the ends of the earth, and break up a brilliant family unit.
I wouldn't even consider that, simply because life has thrown me a curveball - it's madness.
Sometimes things look different to the picture you paint in your head. But you have to work with what you've got.
You've got three children. Your SC need you, and whilst I do understand the yearning for a baby (especially after a late loss, believe me) need always comes before want.