I have 4 children (13, 12, 9, & 6) plus the new puppy... who thinks he is the baby.
I don't think there is much difference going from 2 kids to 3 kids. I think it is hardest with the first one because EVERYTHING is new to you. On the second you know what to do and what NOT to do. By the third you are and expert on everything. Plus, your other children are old enough to help!
If you can afford to do have another one and you feel in your heart it is right, then go ahead. There is nothing greater in the world!!
**The only negative I see is starting the whole baby thing all over again. Sleepless night, diapers, botttles...:D