Using RouterBoard 951-2n as a network extender

I am kinda new to networking and I cannot figure out how to set a 951-2n as an extender. Its a three story house, and I am using 2 of these as extenders, wired by CAT5e to the main router on the first floor. Can anyone help?

how can they be wireless extenders if are connected to the internet with cable?

The best thing is to set the same SSID and security settings on all three routers. Connect them via cable and extend wireless network over them. If you need same wireless subnet on all three routers I would suggest using eoip tunnels.

…or bridge them all