May not be the answer you are looking for, but what I always tell people about the third class.....Why? IMO, the only reason to choose a third class is if for some reason you hate your current class reversed. If you love your P/WL, then great, that is your main, and the character you will be spending time and money on. If you hate Wl/P, well then that is a good reason to choose a third, but if Wl/P works for you too, why waste time/effort/money on developing a third class?
Usually I see 3rd as a better secondary for your secondary class, or alternatively if you don't like your secondary class you have a third class so you can play on that instead of your secondary.
I use my 3rd on Kef as Wd because Warden is fun to play, where as on Beepy I play D/W/Wd, originally I was gonna be D/W, but D/Wd got a buff unexpectedly and Briar shield started giving pdef so I started playing on D/Wd and I do have to say that I do like it although I wasn't really expecting to be playing D/Wd. If you told me two months back that I'd be playing on D/Wd I would have scoffed at you.
Third class is really a gift horse, I would not look it in the mouth.
Anyhow, Ebi would say pick rogue since he loves Wl/R, P/R though isn't very good it has some decent abilities but people don't really play it for a reason. Mage is your other option that goes well with Warlock, Mage/Warlock might not even be that bad, then you'd also have 3 caster classes, it depends on how you want your synergy to work, but if you can avoid stating 3 classes than I would advise you to do so, it becomes a bit pricey.