Pulled from your motherboard manual:
PCI-E Lane Distribution (28 Lane Processor)
PE1 – x16 (x8 if PE3 are used)
PE2 – x1 (Gen2, lane is pulled from PCH)
PE3 – x8
PE4 – x8 (Not functional with a 28 lane processor.)
PE5 – x8
PE6 – x4 (Disabled if M.2 is used)
The HDX 5 is PCIE X4 that supports two M.2 SATA drives - adding in two m.2 SATA drives doesn't change that, it will still be X4. Just make sure that you buy SATA m.2 drives and not NVME m.2 drives (which won't work in it).
Are you running SLI on your three GPUs? If so, my guess is that one of them will be forced to run in X4 mode and break SLI (which requires all cards to run at X8 speed) as you are already using all 28 CPU lanes. Or the card might not be detected at all if one of the X8 lanes running a GPU can't be reduced to X4, which as far as I know should be automatic and can't be set manually.
You might consider picking up a cheap used 40 lane processor off of Ebay. With Skylake-X out, many folks are dumping / have dumped their Haswell-E and Broadwell-E processors. Just an example, but this brand new 6850K can be had for $340, and you can sell your 6800K and recoup some money -
http://m.ebay.com/itm/Int...fffab0e0%257Ciid%253A1