can i use three ethernet ports in one PC. one as a internet port and other two as output ports of ubuntu router

Hello All,
I have a UBUNTU pc with two ethernet ports named eth0 and eth1. eth0 is configured to recieve internet from ISP. eth1 is configured as a router from which i can connect some other clients. My problem is i need two output ports so that i can connect two clients to this ubuntu router in a local network. I can't use a switch or hub with my router because i have implemented some traffic shaping scripts on ubuntu router to the clients behavior on which i am working. If i use a switch an connect my clients to it. these scripts doesn't comes in effect.

so in short i need a ubuntu router with one WAN port and two LAN port. please let me know if it is a valid condition.

thanks and regards
Abhishek