its not just cars, holidays, hotel rooms thats geared to 2+2... even basic stuff like food (pies in a box, slices in a loaf, tinned food sizes, buns in boxes! It all means that theres not enough to go round if you use standard sizes, but if you get double or bigger sizes theres just far too much!) and sofa's, fridge sizes, washing machine loads, dishwasher loads, being never quite big enough, etc.
I know this post makes it sound like im saying its difficult to have 3 children, not at all! Im putting across that life really is geared towards having a 2+2. Of course it is all quite do-able once you get to grips with it (just buy BIG!) plus the fact that as a parent you are led into the whole thing very gently - people dont arrive as teenagers (thank god! would ANY of us have had a second go if they did???), babies eat next to nothing and have a wash load to themselves, toddlers only eat slightly more and are only slightly bigger, by the time you get to 10yrs everything just kind of fits together nicely.
I think i found 3 hard because at that point I had 2 under 2 and an 8y/o with ADHD!
good luck with what you decide - tho i really do think that if you are broody now... it wont dissapear until you feel you have completed your family!