What happens to the motor circuit loading during phase loss will depend on the way the motor is connected and the mechanical characteristics of the motor load.
If the motor continues to drive the load, and if we presume that the motor remains near synchronous speed, then you will have essentially the same mechanical kW going to the load, and thus would expect at least as much electrical power to be supplied to the motor (efficiency certainly won't go up if you drop a phase under load). So if the motor continues to drive the load and you lose a phase, you would expect the remaining leads to supply the total power, and the current drawn on the remaining phases to increase.
It is possible that the motor will stall under the mechanical load, in which case the current drawn on the remaining phases will increase substantially, since a stalled motor draws far more than its normal full load current.
On the other hand, if there is no mechanical load on the motor, then it will probably continue to spin with no difficulty at all. In fact, if you run an unloaded three phase induction motor via two terminals from a single phase supply, the third terminal will _develop_ the third phase. This is the basis of the rotary phase converter, used to derive three phase power from single phase power.
-Jon