I’d imagine that fire mage would be best right now. They do absurdly broken damage in PvM and a lot of their output is aoe, which would be solid in a battleground setting.
Problem is… good luck getting casts off. RBGs will probably be interrupt city.
Those 3 classes have a very different roles in rbgs and are played very different, so it really comes down to your perfered playstyle and what you enjoy. Rogue is probably the most different from the others, so i would suggest you watch a few rbgs pov videos if you never have played it before.
Fire mage is the the first class I tried using to sign up but I got told that fire mage doesn’t fit in RBG’s as good as other casters? (lock,Ele, boomy)
Out of those you listed, rogue is the only one that fits into meta rbg comps. They are extremely high in demand, you cant go wrong with a rogue for rbg. You will however have to defend most games.
I know you didn’t list it, but hear me out. If you don’t mind playing multiple specs (mage gearing may be boring, but is awesome for that, get the same stats and same legendary for everything lol), I’ve seen Frost being somewhat popular for RBG due to mass harassment potential.
Also judging by PTR patch notes, it might be good future investment due to stars aligning in favor of Frost - Fire nerfed so you won’t be judged for seemingly inferior spec, Frostbolt consistent damage buffed, also getting spicy Frost Wall pvp talent (literally a 30yd destructible LoS object), and, very importantly, Warlocks getting nerfed and losing automatic permaslow on Corruption, making Blizzard and Frozen Orb the next best source of mass slow.
I have seen videos of the new frost and arcane skills and the buffs received by frost, also the fire mage nerfs so if things stay the way they are I dont think fire will be very popular if things stay as they are. (I know it’s early days and only the first set of changes)