Have you ever done Xfire or SLI setups before?

That's completely normal, one card heats the other. You wonder why people use raisers and place cards further apart?

You could rig a fan between but even that with 24/7 mining won't be too effective. If you plan on cramming 3 cards, 2 of them will run hot unless you decide to watercool.

80C is not that bad honestly, put all three cards in and mine for a hour and report back with temps you can always rig a fan on top of the cards to help dissipate heat.