I have three. All girls, and now aged 19, 16 and 12, so very much older than many mentioned here.
I always wanted three, though I will admit that the decision to actually go from two children to three was a much bigger one than to go from one child to two. I don't regret any of them and love them all to bits. Financially and in practical terms though it has been hard. Very hard.
We had to move house for additional space, we had to change our car etc. etc. All on one salary because having a third meant a lot more years of being an SAHM than I had originally intended. Indeed, at some points I actually wondered whether I would ever make it back to work, but I finally did it last year.
If you are both happy to have a third then go for it. Your DH's opinion is very valid too though.
We were definite that for us third was definitely final, and my DH had the snip afterwards to ensure that there could be no accidental fourth.
The dynamic can be tricky occasionally, and for us it has been as follows:
DD1 (19) and DD2 (16) are very different. They get on OK with each other, but have little in common really and are not particularly close.
DD1 and DD2 have loads in common and are pretty close despite the 7 year age gap. They both do try to include DD2 in things they do, but although she does join them sometimes, she often just prefers to be one on her own and do her own thing. DD2 doesn't really click with either of them properly.
Both DD1 and DD3 are tops academically. DD1 is already at uni and DD3 also seems to plan on going. DD2 is not academic and will struggle with her GCSEs. She is more practical though than the other two. I think she will be my non-student, and we are currently still hoping she will be able to get an apprenticeship of some sort. Not proving easy though.
That is just an essay about how it is for us, and the considerations you may have to make.