Can someone help me out? Which episodes of season 3 rebels should I watch for lore reasons?
First: the requisite answer you knew you'd get, which is watch them all, they're only like 20 minutes long.
But, I know you're not looking for that answer so:
S3E1/E2 - The premiere, so watch it of course. Introduces Thrawn, etc
E3 - advances the Maul storyline, has Bendu - probably should watch too
E4 - filler, but introduces [new semi-occasional Rebels character originally from the OT movies]
, so it's up to you
E5 - pretty good Hera episode, fleshes out Thrawn some more, adds some Ryloth lore
E6 - filler, but great episode... if you loved TCW, don't skip
E7 - Mandalore storyline begins here.
E8 - Introduces some shitty new characters.
E9 - Hondo. Okay episode, but not essential.
E10 - More Thrawn, slow advancement of the overall S3 story. Eh.
E11 - No spoilers, but DON'T skip it.
E12/13 - Two parter that features a character from Rogue One
that was famously heavily advertised and you already know what it is. Don't skip this.
E14 - Filly Filler. Advances Thrawn a few millimeters.
E15/16 - Mandalore storyline continues. Fleshes out Sabine big time, adds to Mandalore lore. Great episodes. All three Mandalore episodes (3, 15 16) break away from the main storyline, but I wouldn't skip them.
E17 - Shit hits the fan on the Imperial side. Don't Skip.
E 18 - Things escalate even more, and hugely important canon things happen.
Absolutely don't skip.
E 19 - One last filler-y break before the climax. You can totally live without this one, but it's a decent episode that shows some Imperial day to day. Dubious but entertaining resolution.
E 20 - If you like Star Wars, DO NOT SKIP.
E 21/22 - Two part season finale that will air four days from now. You know the answer here. Don't miss it!
Good luck!