I have three, 2ds age 2 and 5 and a dd just 4months... Do it!! Is amazing but be prepared for very hard work! Is easier now my eldest is at school, summer hols were tricky though!
The third child will use everything the others did so don't need to buy more (might want to though!) sell your car and get a bigger older car for similar cost, I have just had a small courtesy car while our big car is in the garage for nearly 2 weeks and am tearing my hair out!
Get a double buggy, cheap from eBay or new and keep nice to sell on after...
My two boys are in one bedroom and the little one is still in with us. Although we have three bedrooms, the boys really want their little sis to sleep in with them, could work with bunks and a bed?
Really is lovely, practically it takes a bit of organising, but once you get used to the fact that you don't have 3 hands, it's all ok!
Last warning, lack of sleep is even harder. As an example, last night ds2 did not sleep till 9pm for some unknown reason, baby asleep at 10pm, baby up at 2, ds1 up at 3, baby up at 4 and everyone up at 6.30, rubbish night but still feeling ok today, so far...
My last piece of advice though is only do it if your dp is happy to, is hard enough without the added pressure of proving to your dp that it's all fine!! Good luck!
We have three. It's mad when it's just me but complete somehow. I prioritised sleep in the early days as I went a bit funny through lack of sleep after ds (nearly three now) and I have coped far better this time. I wouldn't and I guess couldn't have done it til dh firmly on board. We are both bushed but happy at the mo. I have hired my neighbours teens two afternoons after school to be an extra pair of hands and it's a godsend. Don't have three unless you really can't shift the idea any other way. We knew the game was up, after lots of discussion about how crazy it was, when I was in tears all evening after friend announced her third pg.