yes you can do this, what version of windows do you have?
anyways go to network connections, now click setup a home or small office connection, now on the next few screen put "this computer connects directly to the internet" (if this is the computer with the 3g card). now click next. now highlight your main 3g internet connection. click next. now highlight your "Local area connection" the main one that your cat5 cable plug to your computers lan port. ( unselect all others). click next. type in a computer name (it can be whatever). click next. type in workgroup name (this can be whatever you want also. click next. i would turn on file and print sharing but i dont think it matters. click next.
now you will see your network settings. verify everything is right and click next. when prompter select" continue without making a disk. then click finish. viola now you can setup any wlan or wifi enabled device to your wireless router