I have found that installing a card in a PCIe x16 slot that is only x4 lanes bandwidth (like your 3rd slot is) will most often drag down performance of the cards in the x8 lane or higher slots. This holds true for dual cards, and while I haven't tried it with triple cards, I would suspect the same thing to be the case. Add to that the fact that your 3rd slot shares bandwith with PCIe 2.0 x1 and M.2 slot. (if the latter slots are populated)
Hey Grandmaster clutch I had a feeling that was the case. As im going to invest in an M.2 SSD in the future I have removed the 3rd card and will save it in case one of the other two die. 3 vid cards is a bit overkill anyways and the air flow is not great either. Thank you very much for the quick reply.