We didn't decide at all - our 3rd dd took matters into her own hands (don't know how, as we were very careful about contraception!) I was absolutely poleaxed when I found out I was pregnant and then spent my whole pregnancy worrying about all the things you're worrying about now. We moved house when I was 7.5 months, to a smaller house that needed completely gutting, but our old, lovely house had a teeny, tiny garden that was OK for 2 little girls, but would have been hopeless for 3. We bought a 7-seater (which was cheap because it had done 150K, but we've still got it 3 years later as we can't afford to change it even tho' it's on its last legs!) So, in other words, lots of making do, and we felt, in so many ways, that we were going backwards rather than forwards...
BUT - all the worry and anxiety and depression vanished with the arrival of dd3, who is a total delight in every way. The family felt immediately complete, in a way it hadn't with only 2. We are still living in a house that is too small (lovely garden though...) and managing with a car that bits fall off, while lots of our friends have bigger houses, nicer cars, more holidays and a better standard of living. There's no point in worrying about it - it sounds so trite, but dd3 is worth any material loss a thousand times over. I think if you secretly yearn for another then you should definitely listen to your instincts. FWIW, I'm not remotely broody anymore - can cuddle friends' newborns without the slightest twang - and I'm sure I would suffer badly if hadn't had a 3rd. Do still worry about university fees, driving lessons, weddings (a big problem for those of us with girls!!), but essentially believe they have gained more from having 2 sisters than they could by being bought a car for their 17th birthdays. And anyway, have sort of learned that things always turn out OK.
Sorry to ramble on in a rather self-indulgent way, but this is something I feel a bit evangelical about, having been so completely devastated to discover no. 3 was on its way. She really is one of the best things to happen to our family (along with dds 1&2 of course!!)