Transformer

One phase at a time.

With such a sketchy question, your knowledge is well, sketchy.

Read the following article, think about how it pertains to your particular inquiry and try to figure it out yourself. Then, if you still can't figure it out, initiate another help-me post and tell us why you can't figure it out.

http://en.wikipedia.org/wiki/Three-phase_electric_power

http://www.allaboutcircuits.com/vol_2/chpt_10/6.html

Your question could mean two different things:
1) For a load, such as a motor, one phase is lost (blown fuse, loose wire). The two remaining lines constitute a single-phase circuit. This condition is called "single phasing." If the motor was already running and the load on it is moderate, the motor may keep running. But if it wasn't already running, it will likely fail to start.
2) If the secondary of the transformer is three phases ABC plus neutral N, connections to AB, BC, or CA are line voltage single-phase connections; coneections AN, BN, or CN are phase-voltage single-phase connections. The phase voltage is 1/√3 x line voltage.

For an example that is fairly common in the U.S., you could have a 3-phase 208/120 transformer and circuit panel, which could be used with 3-pole circuit breakers to supply 208/3 motors, 2-pole breakers to supply 208/1 heaters, and 1-pole breakers to supply 120/1 lights.