You don't say how old you are. Not that it really matters but if you are in your 20's, I say hold off on donating them.
Your twins are young. My second child is 3.5 years old and my husband and I have gone back and forth since he was about 9mos old on whether or not we were going to try for a 3rd. So basically, what I am saying is how you guys feel right now, may not be how you feel months or years down the road. Yes, you are lucky and blessed to have 3 healthy babies but that doesn't mean you can't have more. If one of you were sure that you didn't want anymore, than I'd probably go with that, but since you are both going back and forth, why not wait 6mos or so and re-visit the topic?
Saying all of that, if I were in your shoes.. I would concentrate on what you have - the twins will be crawling soon if not already, then walking, then running! Imagine being prego during all that! Wears me out just thinking about it! LOL!! I know it's been done, but it can't be easy!
Best wishes whatever you decide!
M.