Those are fairly power hungry servos. If you have them wired up via a breadboard that is likely to be the problem. Breadboards are not intended to carry high currents. Try wiring the servo +/- power connections directly to the battery and also connect the ground to the Arduino.
Also what is the specification of your Lipo, not just 3.7V but mAh and C-rating? Running at only 3.7V is already asking a lot of those servos but if it is a weedy little battery too...
Steve