the term around here is "triandem", even though you can still only nurse 2 at the same time (only two breasts)
but you've got 3 eager birds in your nest, it's triandem!
congratulations on your pregnancy! i wouldn't worry about your 2yo, as long as the oldest is understanding about waiting from time to time. it's a great time to point out to the oldest that he nursed this much when he was this age, etc. it can be challenging at times, but once you get used to it, and develope some type of routine.....it works!
glad to here you are happy about trying. i know you'll succeed.