Ok well that depends. I assume your modem is on the first floor and the wireless signal is weak upstairs. You can run a connection up to it but you would have to specify connecting to that network. Best bet would be getting a switch on the third floor and direct connecting. Wireless points to close to each other can actually interrupt each other. It can be tricky thing to do.
my modem is on the 3rd fl next to a linksys w/ router and i have a line running from the 3rd fl to the 1st fl