Well I haven't assemble a rig for a few years now but from what I'm seeing around AMD Ryzen 3 still have some problem with some motherboard bios, be very carefull with that, same thinks for the AMD GPU especially the newer one.
Note of advice: most game are single tread so core count doesn't matter, clock speed on the single core has more impact.
I suggest as the other an MVM.me driver at least for the SO, for other things if the price for the MVM.me drive is to hight looks for a standard SATA 3 drive: we are at a point where you can buy a 1 TB SSD for 30-50€ (for storage you don't need somothing lake a Samsung EVO, just grab a Kingstone and you'll be fine).
Olso a suggestion: if you plan to keep the rig for several years invest in it now: I was used to buy medium cheep GPU and upgrade later to the same laver of the new generation, well making bottom line count on a 6-8 year time circle it will cost more then buy the top line (or the 2nd rakned GPU) at the start and you also need to get in compormise with performance. It will cost more upfront, but it will pay in the long run.
Dont bother with a multi GPU setup: those are only for test bench and doesn't relly affect your gaming experience.