What brand of belts/tensioners does AutoZone carry? It sounds like a bad/incorrect belt to me.
The correct Goodyear numbers for your car are:
Belt: 4060695
tensioner: 49205
Let me know what brand they carry and I can cross-reference their numbers to be sure you got the correct parts (the same part numbers apply to 96-99 Luminas with the 3.1) even though you said you double checked.
Assuming the parts are correct, you could have a wrong compressor as stated above that has a smaller diameter pulley causing the belt to be too long. On the belt tensioner, their should be an indicator showing if the belt is in an acceptable range. The tensioner arm should have a notch or pointer on it and the body of the tensioner should have a range indicator that the notch should fall in if the belt is the correct length. You may need to get a belt a little shorter than the car calls for if that is the case.