### First of all: "Flashback" would be when you replace an installed BIOS with an older one. I'll answer the question the way I suspect it's actually meant. ###
According to https://www.msi.com/Motherboar..., the board supports the 3700X from the "M0" BIOS. This is the BIOS version with which the board is delivered. Since my 3600X (which was released at the same time as the 3700X) had no problems with the M0 bios, the answer (in all likelihood) is: No, no flashing necessary. You can install it straight away and get started.