Well, we were just beginning to even discuss if we wanted #3 or not, and we found out that was no discussing needed! We have a 3 year old girl, 5 month old girl, and I'm 9 weeks pregnant with #3. Needless to say, it was a surprise! I too, I have all your same concerns! I'm barely learning to deal with two, how will 3 work ... especially with 2 only 12 months apart!
Some practical things you might want to think about as you make your decision. Is your house big enough? We barely had room for #2, so we're in the process of selling ours and trying to buy another one all before #3 comes. Also, do you have a car big enough for 3 car seats (remember, your oldest will have to be in a booster longer with the new law)? We don't, so we're going to have to buy another one before January. These situations are adding a lot of stress, both mentally and financially, to our life right now, but we know it will all be worth it in the end!
As for being pregnant while taking care of two kids -- I'm exhausted! Computer time has been valuable for me! I save my oldest's 30 minutes for the baby's afternoon nap and then I can at least rest while they are both otherwise occupied. I've been told by numerous people that after 2, adding another isn't a big deal. I hope this turns out to be true! Good luck as you make your decision! I hope my ramblings have at least given you some things to think about!