Its called being wired parallel.
if the batteries are of the same specs, and you have charged them seperatly to full charge, you can keep them connected for a long time, just treat them as if its 1 big battery.

Best bet is to give them seperatly a top up charge once a while.

If the batteries are 3 different sizes and/or with differences in quality/wear and tear, then its 1) not a good idea, 2) you need to remove and charge them seperatly