Just a standard XLR to 1/4" jack cable would do the trick. You should be able to get one from any music store.
I could be wrong, but I have a feeling it's only the clean signal that is sent through the XLR out on a B3.
Can't recall 100% at the moment.
'80 Rick 4001. '84 Fender Power J Special, '05 USA Jazz, '11 USA Precision, M Miller Jazz. Peavey T-40, Zoom B3. GB ShuttleMax 12.0, GB Uber410, fEARful 212.