Did you buy the MSi board already? If not, recommend you look for another brand as the MSi MPG boards have terrible power supply designs. It should be fine with the 3700x as it is only a 65-watt CPU but the boards cannot handle 105-watt CPUs as they will run really hot. That would mean if you wanted to upgrade to a 105-watt CPU down the road, you will have to buy a new board.
Regarding your initial question again, it should be fine with the 65-watt 3700x. My MSi X570 MPG Gaming Carbon Pro is only using the single 8-pin power connector with my 105-watt 3900x. The board does generate a lot of heat, up to 95 degrees C under load which is far too high when other brands can be as low as 60 degrees (most boards run between 70-80 degrees C).
I have seen my board throttle the CPU speed down when it hits 85-95 degrees under load (when I am encoding videos which is a lot more CPU-intensive than when playing games). I don't know why it throttles at different temps but it is discerning to see the CPU speed drop from its average 4.2 GHz down to 2.2 GHz when it gets too hot. I can see the effects when the encoding time jumps from 40 minutes to 60+ minutes as the CPU speed goes up and down.
I did report this issue to MSi and gotten nowhere. They said that if the CPU is throttling, it is a defective CPU. My only recourse was to buy the $350 ASUS X570 ROG Crosshair VIII Hero board that has a lot better thermals.