This is a tough question, often money is an issue. However since you mention being able to aford anoher wonderful. Now, as you have prolly figuredout LOL that having small children is a lot of work, not to mention the noise factor. My husband comes home sometimes and there are toys, noise and cartoons. Sometimes he says he would like to come to a quiet home, and sometimes that does happen ( nap time) but not so often. The more kiddos you have the more noise. Also did you have complacations during any of your pregnancys? I had two repeat c sections ( babies were too big for me). Also are you ready to handel the what if your child has special needs and also the day to day with the other two?
Having said all that, I think having children close in age is wonderful, maybe discuss the possablity of having another child that might turn out to be a boy. ( we have two boys and the seond was a try for a girl, he has special needs and decided we would not try again for a girl) Also ask what his concerns are...keep talking about it and maybe wait till you can get him on board. Having two kids is one thing but with the addition of another there are things like a bigger car, getting around ( family is great help for this) more feedings ect.....
Do not give up you, having a big family is nice exspecially when they get a little older...dance class, T ball.
I guess the bottom line is to get your husband on board and if you can't then maybe should wait. You know his feelings count too and I totally understand your wanting them close, our boys are only hmmm 10 or 11 months apart ...almost like twins in away.
Good luck