what if my source is a 3 phase AC then my load should be purely resistive, how do I match the resistance/impedance to get maximum power. (direct loading)

 

Are we talking about three resistive loads, one for each leg? I don't think the maximum power theorem has anything to do with whether the source is a battery, transformer, solar cell, generator, amplifier output or three phase supply. Maximum power is delivered to the load when the load is equal to the source's internal impedance.