In Alternate Current(AC), you can see phases. That is the phase what we actually cal 1 phase. In 3 phase, there will be 3 lines carries AC with phase difference of 120 degree i.e., 1 complete cycle of AC (360 degree) is divided into 3 parts= 120 degree each. so that, in 1st line Magnitude starts increasing at 0 sec. when it reaches 120 degree, in 2nd line Magnitude starts increases from 0V. while 2nd line reaches 120 degree, 3rd line magnitude starts increasing. So that, the instrument to which 3 phase current is applied will get max power to drive.