The best solution would probably be to use a pair of powerline adapters, one near the router, the other on the third floor, thus using the power lines as a bridge. You could then patch a wireless router in AP mode to the upstairs powerline adapter and create a local wireless hotspot.
[wireless router]<-- wire -->[pl-adapter #1](ac)<-- power lines -->(ac)[pl-adapter #2]<-- wire -->[wireless router – ap mode]<-- wire/wireless -->
IOW, everything in BOLD is a substitute for an ethernet cable.
In this setup, the second wireless router doesn't need to be anything fancy, but for good performance, I wouldn’t use anything less than 500Mbps rated powerline adapters.