Well, there is a tag added to the Hero when it becomes an ex-member of a class that you can check for (ExMember.cHelpPal, in this case).
And there is also a tag added for each level of a class added to a character (Classes.Paladin, in this case).
I haven't worked with custom classes, so I'm a little fuzzy on the specifics of how their abilities should be bootstrapped, but you should be able to test for the existence of the ex-member tag and number of levels by using a bootstrap requirement something like
hero#ExMember.cHelpPal & (count:hero#Classes.Paladin >= 3)
http://hlkitwiki.wolflair.com/index....itle=Tag_Terms