Log on to wireless router's configuration page, configure following settings: LAN and wireless settings.
IP address 192.168.0.254 with netmask 255.255.255.0 (if you use XP/Vista on that computer with 3G)
IP address 192.168.137.254 with netmask 255.255.255.0 (if you use Win7 on that computer with 3G)
Assigning the mode, SSID, wireless channel, SSID broadcast and other settings as according to your needs.
After that enable Internet Connection Sharing on dial-up connection, connect this computer to router's LAN port with network cable. Finally test to connect wireless by using other computer/wireless device (need to configure to obtain IP address automatically).
This is an example to configure wireless router as access point, can use as reference.
Hope this helps.