M.J.
In a nutshell, you go from a man-to-man defense to a zone when you have your third. Somebody will be left out, there are only 2 grownups in the house. There's also the car issue, some cars can't fit 3 carseats in the back, so you have to have a bigger vehicle. There's also the vacation factor, with 3 kids you might not be able to get a regular hotel room and might have to get a suite.
I have 4 and my first 3 were very close in age and it was VERY hard. Now that they are older, it's good that they are so close in age since they go through the same stuff, like the same kind of video games, etc.
As far as getting slim, it took me about 4 years to get my body back after the 3rd one.
I don't have any family in the area to help me, so driving and going to sports events can kind of be tricky as well as doing stuff at school. You don't say how older your other kids are which to me is a big part of the decision. Maybe you don't have to worry about the carseat thing, maybe you have a built in babysitter with another sibling, both of those would help.
But if you are concerned about the amount of attention your kids get, you want to think about it because there will be 3 of them and only 2 adults.
M.