Remove two of the memory modules and try booting with a single kit after you clear your CMOS.
Check your mobo user manual to make sure that you use the correct memory slot.
You will have a much easier time if you buy a single kit of 2 x 4GB or 4 x 4GB memory and use your old tri-channel memory kit as a high tech paper weight. Obviously you can sell your old memory to help offset the cost of your new memory.
You can select memory from the Gigabyte memory QVL list for your motherboard or buy a kit that the memory manufacturer has tested with your motherboard.
Non listed memory kits might work out OK, but they might require more effort to ensure that your system is stable.