Hi Carl,
Typically, CoS 3 is call signalling and CoS 5 is for voice traffic - in Cisco devices anyway.
Unfortunately, there's no specific standard (that i'm aware of) which specifies what values should be used at layer 2 or 3.
In reality it doesn't really matter what values you use, as you can configure the switches and routers to do whatever you want with specific values.
Michael.