I think you have to do what is right for you, and your family. It is a decision only you can make.
Are you feeling able to go through pregnancy again as quickly, health wise? Can you afford another child, is there enough room, size of house, car etc etc. 3 means 3 carseats, more expensive holidays, meals, so many things cater to a family of 4.
You will have 2 very young dcs at the same time which will make it tougher, not to mention being pregnant with two young dcs. On the other hand, because they're little you'll get it all out of the way with quicker, you're in the hands on years anyway, it'll not be much different time wise. I think this is easier in some ways than going back to it later on, and the sleepless nights just dragging on for more years. Your dcs would be close in age meaning you can plan similar activities too.
You have to do what is right for you, when you're in the thick of it things can seem impossible. However later when you're getting sleep again, as your other 2 grow will you regret it?
I have 2 dcs, and dh has now had a vasectomy, I just know I couldn't cope with a 3rd. This is because my dcs are 7 and 4, we have more freedom, doing more activities, easier outings etc.
I wouldn't want a baby thrown in the mix now. I am in my late 30s, and just getting my identity back. If I had have fell pregnant when dc2 was 1 and had an abortion, I would regret it now. I would be thinking the 3rd child would be more manageable, we would have done the hard bit, and this little person is missing. I would spend my life wondering what if, what they would be like.