Powering the servo rail

All four ESCs on my quadcopter came with BECs, can I connect 3 of them to the servo rail using only the ground and the signal wires and connect the fourth ESC with all 3 wires to power the servo rail?

Yes, that is correct way to do this.

when do i need to power the servo trail ? just in case i use additional servos, or do i need to to this also to power my ppm receiver ?

many thanks in advance

@0xCCD RC receiver is also powered from a servo-rail, so yes, in that case too.