No, judging by your discription of your mobo, I don’t think it’s supported, usual pcie support 3 way sli can do either 16-8-8 or 8-8-8 on the pcie land, I think the mobo supports crossfire. If you can tell us the mobo model, we can help you more
Here is what i think, and what you need to check to see if it’s able for 3 way sli.
1. The 4x will bottle neck the card for three way sli.
2. Latest Gen 10 only supports 2 way sli. You will need Titan for 3way.
3. Just because the motherboard has pcie slot doesn’t mean it’s able for sli, check the mobo manual to see what mode of sli is supported.
4. Not every card for Nvidia can be used for sli. You have to check you card to see if sli is support or not as well.