Three kids is really hard, but of course can be beautiful too.
This sounds like terribly flippant advice but I think works.
Go over your pros and cons, and then flip a coin, eg heads keep, tails end pregnancy.
You don't have to decide on the coin flip, but as it lands your instinct will tell you if that's the choice you're happy with.
I have had terminations and found that what I grieved for wasn't the baby I wasn't having, but the life I would have had to be living in order for another child to be a viable option (more money, bigger house, not having to work, etc etc).
I also marked what would have been the due date as a 'family as it is' day and appreciated the family, grateful that I didn't also have to deal with a newborn on top of everything else!
I've given these ideas before you may well find me on other threads of you are reading old ones for advice!
Good luck whatever you decide.