This as simple as I can put it... If it says X58 or socket LGA 1366, it utilizes triple channel memory (3 sticks or 6 sticks). All other MOBO's utilize dual channel memory (2 sticks or 4 sticks).
Core i7 920, 940, 950, 965 & 975 are the only ones that utilize triple channel memory at this time. Moving forward, the Core i9 CPU's will utilize triple channel memory when they are released next year.
By the way, how would I know if the mobo needs at least 2 or 3 memory?
You don't have to have 3 DDR3 sticks to run with. You can have just 1 slot filled if you like, so you don't have to buy 3 sticks to "make things work". It would be a shame to only run with 1 or 2 sticks of DDR3 on an i7 mobo, but it'll still work. You wouldn't take advantage of the "Triple Channel" that the i7 mobo's support, but it will work without 3 RAM sticks.