Cars. Laundry. Hotels. Holidays. Air Fares. Money, etc.
But really, time, money and logistics.
With three there are no free evenings. One wants to do gym, one wants to do dance (three different sorts), one wants to do football and rockclimbing. Cubs and Brownies are on the same night in different towns. (Actually, hold that, at one point I had one child in nursery, one at Infants, and one in yr 3 juniors - I thought I was being clever having them all exactly one year apart. The schools were in different towns with 5 minutes between bells) Two of them want to do swimming lessons so the third one does too - so you end up at the pool from 4pm through to 6, sweating buckets, and trying to keep the other two still and quiet whilst they aren't in their lessons.
Weekends are destroyed - you imagined lovely family time, but what you get is at least one sporting/ extra-curricular that clashes with a birthday party. You aren't quite sure who your husband is, as you are usually at opposite ends of the county/ country dealing with one or other child.
Um, and in our case - number three was brain damaged at birth, which made the first few years a bit more complicated...
But we now only two (over) 7 seater cars, and only ever holiday in self catering apartments or camping. We're financially skint, but emotionally rich... probably.
I'd quite like another, (we have the infrastructure now ) but DH has been well and truly fixed and won't countenance a reversal...