I have a 3-story house where my router is installed on the top floor. I would like to have WiFi on the bottom floor. I've got some sockets in the wall for TV and telephone but not for WiFi.
If I may, you can try a POWERLINE adaptor. It turns the standard AC power outlet into an IP network using existing copper in the house. Plug one into the nearest outlet by router, run cord from router to device, then go to the nearest power outlet where you want your PC, and plut into that outlet. Now you have just extended you network, without the use of WIFI. Take a look here.
Extender is the best option for your problem. There are lots of wireless extender in the market. You can use one of them. But i personally recommend Netgear wifi range Extender to you. This is best.
So where is the extender going to be placed? Doesnt the extender need a wire too? So how is the OP going to run a wire to third story? And if they do, why not just ise that LAN?