Power supply is -way- too small for 3 Vega cards, which can pull over 300w each themselves, you're going to need a 1500w unit since they can even draw 350w under a heavy load. Because they drink power like its water, they throw off an insane amount of heat, and need a lot of airflow, which is something a single small slot between them cannot handle unless you have high airflow fans pushing in air to them. Then there's the problem of your motherboard
which, at best, is x8/x8/x4, and x4 is insufficient for a card of the power of Vega 64. Then there's the problem of your CPU, it's far too weak to play games at the resolutions and detail levels you are using 3 Vega 64 cards for, which I assume would be, minimum, 4K high details.