If the PC Of K3 is that weak then I have to ask why is he/she going on that quest in the first place.
Seriously the writers of KOTOR 3 are going to have to come up with a pretty good reason
as to why the PC of K3 is able to defeat the True Sith or why he/she is the only one who's making a concentrated effort to stop them (if they exist) and yet more experienced and arguably stronger people such as Revan, Exile,Bastila, and I'm sure various others can't or aren't.
In the words of DM Of the Rings: If you're such a badass why don't you do it yourself?
And having HK, Candalore, and any other character aside from maybe T3 and Basty be party members again would be redundant because quite frankly I think most of them are old hat and them joining you again for no apparent reason would be stupid.