and place the wifi router, nearer to the centre of the area you wan to cover.... such as near the basement door(?)
also you might want a strong antenna for the laptop as well..
Other possibility is move your WiFi router to the second flr(?) and check if the basement pc and the 3rd flr pc get an acceptable signal (You need a cable for this..)
Then you might also try to use a cable again from the wifi router to the 3rd floor and use a range expander.
Basically, a WiFi should have a direct line of sight, to have the maximum range.
In my place, I setup a cable from the Wifi Router to any room that might need a connection, and if I need a mobility, I just plugin a 'repeater' on the cable. BTW the Access Point is placed in the center and open area of the house.
I have a Linksys router w/speedbooster in my basement, next to my son's desktop, which was the "host." My other son's room is in the 3rd floor, & he kept losing his connection. We tried a Linksys signal booster, but didn't work, so we solved this by hard-wiring his desktop to one of the ports in the router. NOW, the problem--he has Dell laptop with Centrino which he uses for school, & we're back to the old problem, the signal to his room is so weak (or non-existent)that he can't download or e-mail files to himself so he can access them on his desktop. He's been using a "memory key" to transfer the files, but gets no internet connection. Linksys recommended I get a wireless access point (WAP54G)to "extend" the router's range without losing signal strength, but this product did not get very good reviews. Another Linksys tech recommended the WRE54G which "extends" the signal,but cuts the strength in half. A friend suggested an inexpensive "hub" to connect his desktop to his laptop, but this means wires & less portability. Any ideas/suggestions that will extend the range or boost the signal. We have 2 desktops & 2 laptops, all running on Windows XP.