Check with them ahead of time to find out what type of outputs they have on their board. They will have some combination of either xlr, rca, or 1/4" outs. You can get or make cables that will then fit the 1/8" imput of the JB3. Do you have any type of pre amp you'll be running in front of the JB3?
You might also want to look into attenuators which reduce the signal coming from the board. If you're just recording straight to the JB3, even with the internal gain all the way down the signal can still be too hot and ruin the recording.
and the other times they are going to be in bars
Do you mean that they won't be using a PA? Just like amps only and no mics for vocals?