No, for my MPLS circuits I am not using the WAN port as there aren't ethernet modules there. Also, I've taken my 3120 off my MPLS and upgraded to a SR4134 now.
I do have one non-MPLS connection on a SR3120 pair that is a DS3 circuit, and you are correct, instead of referring to it as "interface ethernet x/x" you say "interface bundle yyy" and then "link t3 x/x" or "link t1 x/x". However all of that is just to identify the port you are configuring, once you get past that part, the rest of the commands (ip address x.x.x.x, etc) are pretty much the same.