can you answer my questions?!?

Ok i have 2 questions here.... when it comes to stereos i dont know all too much but enuf to get me by..... but my questions are:

what would happen if i was to connect 3 subs on a 2 channel amp?? does that distribute the power to all 3 subs?? or does that change the ohm settings??? reason is cuz i want to add another sub but i dont want to go for another amp, since i already have 2.. so i need to know what would happen if i added another one to the amp...

the second question is whats this ohm thing??? how do you set different ohms like 1,2,4 ohms?? and why is this important???

thanks for your help people...::