I have 3 daughters, ages 5,2, and 3 months.  I also debated as you are and asked all the parents I came in contact with who already had 3..what it was like.  The answer I most often received was that going from 1 child to 2 children was the most difficult...going from 2 to 3 children was easy. 

I have to agree with all the parents who said that.  When you go from 1 child to 2 children you no longer have the option of ..here honey you take the baby for a bit...I'm tired.  When you have 2 it's here you take this one for a bit and I'll take that one...and see if you can find a break from a change in personality LOL

When the 3rd one comes around you are so busy anyway that putting one more kid in the tub..or making dinner for one more really isn't that much more work.