highest 2 points for b datum plane and highest 1 point for c datum plane

Assuming this is an assignment sort of solution if not using Geotol. I like to use legacy so I can visually see what is going on and control everything myself instead of using Geotol where I have only a slight idea if its doing it correct or not. When I have 3 planes as a datum, I assume that when I label them A,B and C, that Geotol is taking the 3 highest A datums points for the plane, the 2 highest B datum points from the plane and the 1 highest C datum point from the plane for the alignment. Doing an alignment I use "Primary Datum" for the A datum plane. Out of 9 points, 4 of them are 0 and 3 of them are in the negatives. This tells me its working correctly. But for the B datum and C Datum, its taking the centroid for the alignment. So half the points positive and half negative. How do I get these planes to align off the highest points/point.

All planes are perpendicular to each other. Its an L bracket. Front of L is the A datum, bottom of L is the B datum and 1 side is the C datum. I'm using 2020 R2. Some times the b and c datums are tilted differently from part to part. So basically want to mimic pushing all 3 planes up against blocks. 321

I have minimal experience with assignments and such, be gentle.