I just built a new PC but when I start it I get 3 long beeps and o signal to moniter. From what I've found while trying to solve this it might be my mobo doesn't support my RAM. I have an MSI H110M ECO mobo and 8gb ddr4 - 2133 UDIMM for RAM.
if you have tried it with gpu removed and display cable plugged into motherboard... it's either ram or motherboard. you said you checked and your ram is compatible and seated in the correct slot? if that is the case, send the motherboard in/take to computer shop and see what they say