You can buy a WiFi access point, and wire that into one of the Ethernet ports. Locate that access point where the signal is weak.
You can use the same SSID and security on both. »superuser.com/questions/122441/m···ame-ssid
Do not have the access point do the DHCP; let the 3800 pass out the IP addresses.
Alternatively you could use a different SSID, and select which you want to use for each computer.
A wired access point will be better than a WiFi range extender.
If your 3800 is at one end of the house, you could position a sheet of metal, possibly in a corner reflector configuration, to improve range in one direction at the expense of other directions.