I have 3 tables A, B & C each of which has a 1-to-many relation with a Comments table but I can't fathom how to set up the foreign key(s) in Comments without resorting to distinct A_id, B_id & C_id columns in Comments and enforcing 2 of the 3 columns be NULL for a given row. Seems ugly.
You should probably take a look at inheritance and see if it can accommodate your use case.