I've enjoyed reading these responses...very interesting. I am a member of another parents network (based in Atlanta) where this question was recently posed, so I'll chime in with what I recently read (it's only what I read...I have no personal experience with 3!). I think the local mom's are a lot more encouraging than the responses I read on that network!! (Kudos to you all for making it work!!)
Most of the people responding said they love their 3 kids but wished they stuck with 2 or went up to 4. Mainly because with three, two would always pair up leaving one out (and requiring more of parents help fixing things as the 1 wouldn't have anyone to go to). It creates a single middle child who can feel lost. Those who had 4 said it was much easier than when there were only 3...as they can pair off in two's (one is rarely left out).
With that being said, the ones whose 3 kids are older said it's great now but definitely more a challenge in the younger years...might have done things differently, but no real regret (because can you really look at your youngest and say that you wouldn't have him/her?!).
Most had no regrets of having a 3rd (although a couple admitted life would've been easier sticking with two) - the majority said they wished they'd gone up to 4 instead of stopping at 3! I was surprised by that!! :) (That thought scares me - in spite of growing up in a family with 4 kids!)
It was split where some said the biggest transition was going from 1 to 2, and others said the biggest transition was going from 2 to 3. Personally, I think that this really depends on the situation with the parents (are they both working, how much do both do, etc) and probably also has a lot to do with the age of the oldest one(s) when the next comes around.
Really, your oldest would be at a prime helping age by the time the baby was born, and your son is at the stage where he's a little more independent and probably want to 'help', too. You're a SAHM, which means you're there with them...so no added day care costs to really consider. My son is also 2 and we've talked about another and both end up feeling the same pros/cons (about wanting another, yet my son is just getting to the point where things are becoming 'normal' and he'll be potty trained sometime soon...do we really want to start over again?!). :) Best wishes in your decision!