i am currently having an arguemnt with my dm i want some answers how many powers can a psyker cast in one turn during combat is it only one power or can it be 3 if it were to be two half actions and one free action, or is it jsut one power even when it is a free action or a half action

Only One Power, per round.

Only exception is the use of Resist Posession, which may be used as a Reaction even if you have used another power that round.