If at the intake position from the utility company it is three phase, and the load is, say 50amps per phase.

Would the return shared neutral take that full load of 150 amps? Or is there a reason why it would be lower because shared neutral conductor size is always the same as phase conductors.

I know single phase does take full load return

The three phases are, (let's see, how do I say this without sounding dumb? Oh well -- ), "out of phase". The currents cancel or partially cancel each other out. The return neutral does not carry 150 amps.

If you had two waves exactly out of phase (180 degree phase difference), they add to zero. This is a bit more complicated since you have three phases 120 degrees apart, but the effect is the same. If you're not using the phases equally, you can get up to the maximum of one of the phases (50 amps in this case), but that would be rare.