ADSL MODEM PORT FORWARDING

At my home, I have a DLINK dsl-504 adsl router. The router has 4 ports, and there are 3 computers connected to it. When i go to configure its settings, i find extremely irritating that i can only forward/open a port to one ip address at a time. Therefore, i can only have internet access with certain programs on one of the 3 computers at a time.

Is there any way to get around this at all? Please help me!