Try one or more box fans behind the rig first - you need airflow above & beyond the GPU fans.
Then try one card at a time, then two, and so on. There will be airflow interactions, so watch what happens to all the board temps.
If you find a 'hot' card at stock speeds consider RMA - it could have poorly applied TIM, but RMA is for before the board turns black and bursts into flame
BTW - crossfire is usually a no-no.