The GAMING 3 has two M.2 slots while the GAMING K3 only has one. The GAMING 3 also has a whole lot more PCIe lanes available for use. You could use two GPUs, a PCIe x4 card like an SSD for three blazing fast PCIe based SSDs and then three PCIe x1 cards like sound cards and such.
I would go with the GAMING 3