Golden Frieza, BSSJG Vegeta and Goku vs. Beerus

Beerus is not holding back.

Are we assuming that the trio are actually functioning well together as a team?

Yes.

Team wins in that case. The two saiyans alone were said to be able to take him

Team should win, then.

Whis stated that if Goku and Vegeta worked together, they'd be able to match Beerus. Adding Golden Freeza to the roster should tip the scales in their favor.

This is how I see it:
Golden Freeza(100%)+BSSJG Goku+BSSJG Vegeta > Beerus = BSSJG Goku+BSSJG Vegeta > Golden Freeza(100%) > BSSJG Goku = BSSJG Vegeta > Golden Freeza(depleted.)

So yeah, the only thing that might hinder the trio's victory is if Freeza were to become significantly depleted before Beerus is down. If that happens, it's anyone's ballgame.