To be more precise/specific:
Several N64 (definitely not the majority) games run very well on Pi3B+. For some less demanding games like Mario 64 even a RPi2 is good enough.
Regarding PSP and Dreamcast, there are only a few less demanding games, that run well and some run okay, but not 100% fluently on RPi3B+.
So it's a kind of hit&miss/try&error for the less demanding games.
If you want best performance with all (or at least the majority) of N64/PSP/Dreamcast games, then you should definitely pick a PC instead of RPi3.