I've had a III since they came out and it should work fine. If you want to go into the effects return you'll have to put a plug in the input.
I just used an old switchcraft I had laying around and ran my G3 and/or a TEc21 pedal with my M-13.
Or, you can use the Flex by itself if you have drive pedals. The included effects are not perfect but they work pretty well live.
I do have the FB for it though. I bought a Mustang V and so far it hasn't beat out the Flex III. It's getting closer.