Datum B is 2 points with car coordinate system. How to setup and verify the datums?

That's a standard 3-2-1 type alignment. Level and origin to -A-, rotate and origin to a line created from B1 and B2, and origin to -C-. You need to follow the order, with -B- as the origin in 1 axis first, and then -C- origin in the other. 

Yes, this the basic. I usually rotate the CAD first, move 0,0,0 to datum C in this case. And calculate B1 B2 locations by subtraction off datum C's

Datum -C- is not the origin in both directions and not 0,0,0. There is no calculation needed. You need to rotate & origin to -B- first, then origin/translation to -C- for the remaining unconstrained axis. 

Ok, I see. 

For me, I usually rotate/move the cad origin to center of datum C. sometime I have to rotate the coordinate system to have part sitting position vertical or horizontal. 

Then I will use read point to get center of Datum C and center of an other hole close by. With 2 points read in, I could easy start DCC with circle C , then plane A , datum B is the last one, Since I use read point, I need to calculate B1 B2 locations. 

I think you will manual check 3-2-1 with Datum A B C ...