Worth completing LS2 and 3 for gems?
LS3 is more worth it because of the ascended trinkets
And the additional maps and currencies, minis, potions, etc., if you are into that sort of extra content.
Good luck.
are there only certain chapters of ls3 that are useful, or do you really need the whole thing?
I’ve heard lots of people mention farming the Ice chapter (3 I think), and that is one of the 3 that I have.
Episode 3 is probably the most useful with episode 1 being the least. Having all of them allows you to generate enough map currencies to get ascended trinkets much quicker.