My three are 9, 6.5 and nearly 3 and it's busy! Sometimes I feel like we live in the car with school, activities, sports, sleepovers, parties etc... and as a pp mentioned, the washing never ends!
That said, my youngest has always just fit in with the older two and we've dragged him around the countryside without him giving us much grief. And they all get on really well - the older two do have their squabbles but most of the time they're good mates and all three are happy to go off together and occupy themselves.
I don't want to make it sound like rainbows and lolly pops because there were definitely tough times - Sleep deprivation, the juggle of dinner, bath and bed - especially because DH is often not there to help, but you do get into a groove. I love having my three and if I could, I'd happily have a fourth!