Okay ... the ESC will power the servos, it does not matter if you use 3S or 15S the ESC can provide the correct voltage (unless its an OPTO ESC where you will need a separate RX batt). The issue for bigger set ups (4S and above) is that the motor draws so much juice that your servos will not be reliable. For 3S set ups you do not need a UBEC (a device that draws power from your lipo pack for the RX) or separate RX batt. I think the ESC only gives like 4.8V-5V to the servos. This is how many many people are flying EP planes and no one has BBQed their servos.