Ham Radio Antennas, Loading Coils, and Third Harmonics

I am in the process of rethinking my attic antenna setup. Realistically, I have space for, at most, two HF dipoles running parallel and a 2m/70cm antenna crammed into one end. That got me thinking about two things: loading coils and third harmonics for additional band coverage with the same antenna.

I can find instructions on how to build a loaded dipole well enough. What I can't seem to find is a handy list of third harmonics. I have the ARRL Antenna Book and the ARRL handbook. That information is probably buried in there somewhere, but I can't seem to find it. Is there a handy chart somewhere?

Is there a generally agreed upon cutoff for having too much loading coil and not enough antenna for SSB voice comms on a given band? Thanks!