If you can swing it, get the Gaming K3 board. Much better in my opinion. That would have been my first choice except I gonna get the Gaming 5.
The hd3p has pci slots which really are old technology now, whereas the k3 has all pciexpress. The hd3p has onboard vga output so if you have an older monitor you may need it, the K3 only has dvi.
HD3P vs K3: Well they both have two PCI Express x16 3.0, so aren't they both good in that aspect?
I'm using DVI anyway so that won't be a problem.
You mean Gigabyte's Z270X Gaming 5? Besides the LED lights, what more does the Gaming 5 have compared to the K3? Just the 3-GPU thing?