How long are you leaving them when you test them? May take a few hours to get them all the way heated up. Are they all plugged into the rheostat at the same time? Throw all three cords in a powerbar, plug the bar into the rheostat and then let them all heat up, then check temps. That's my setup exactly since my thermostat failed and mine are all even (give or take a degree or two as you move up the rack).