Prioritization between two local interfaces?

Question

linux

tell me how to do it right:
there is a router built on a PC with debian 8, with 3 network cards:
eth0, 10.128.3.1 - local network (asterisk 6 external lines, 10.128.3.2)
eth1, 62.1.1.190 - provider (40 megabit / s )
eth2, 10.128.2.1 - local network (samba, squid, 10.128.2.2)
how to guarantee bandwidth for eth0, but if there is no traffic from eth0 (asterisk), give eth2 (internet for users)?
For me, tc is a new thing, I'm just learning. Everywhere they write about the possibility of balancing outgoing traffic on the local interface, and I have two of them.