It depends what else you want to do at the same time. A few browser windows, or listing to a podcast / youtube video in the background or something isn't going to be very demanding on the CPU.
If you want to stream your gameplay, or do certain other things, then you would certainly want a CPU with more cores.
If it were me I would probably wait for B550 for PCIe 4.0, and go with the R5 3600 as it is probably more futureproof thanks to the additional cores - even though performance in modern games is fairly similar.
If money is an issue, however, and you're not streaming, just gaming at 1080p 60hz then an R3 3300x + B450 board will do fine.