|You need to know the LAN side IP of the 3Com router. |
Once you know that, ensure you're plugged into a LAN port on the 3Com with yoru PC and then open a web browser and type that IP in the URL field and hit Enter and it will take you there.
From the sounds of it, your 3Com is downstream of the Vonage. How is it connected exactly:
1) Vonage LAN Port >> 3Com WAN Port
2) Vonage LAN Port >> 3Com LAN Port