I have a part that is shaped as a parallelogram. Clearly it makes sense to have the flat surface (plan view of parallelogram) to be datum A. One edge is datum B.
Can I make a 3rd edge (which is not perpendicular to Datum B due to parallelogram) the tertiary?
GDTguy nailed it. Most people fail to distinguish between Datum Feature & Datum ... the first is the physical feature on the part, whereas the second is a theoretical (perfect & infinite) plane generated by the Datum Simulator.
There's nothing wrong with using the entire third surface as the tertiary datum feature, depending on the functionality. The corner of the prallelogram will make contact with the tertiary datum simulator when the surface is inclined at an angle less than specified, otherwise it will make contact with some other point on the surface.
Using a datum target makes it more sensitive to actual surface irregularities (less repeatable) than using the first point of contact with the datum simulator unless you use a movable datum target ... a small deviation in the angle of the actual surface can make a huge difference in the actual contact point on that surface (particularly if it's a datum target point).