It's not about PCI lanes, the same X99 WS motherboard which runs 7x 1070 or 7x 1080 Ti's will not run 3 Duo's unless you use the newer AMD drivers which reduce the mining speed by half.
Have you tried any of the newer versions OTHER THAN 16.12 series, which had KNOWN ISSUES on all cards in a mining setup with poor performance?
There's a reason I specified the 16.10.x series (and the WHQL version in particular).