Thanks Vann!
Great tips. I had tried the fan method but that did not give me 100% certainty, the cards being quite close together.
I guessed you are using afterburner to increase your GPU fan speed, and you have the same series of GPU (at least 2 or 3) It must be, this option "
Syncronize setting for similar graphics processors" checked. That's why your GPU that has the same series (similar graphics processors) affected by one of GPU setting.