First, are they wet cell lead-acid batteries? Most (but not all) lead-acid batteries in common use are either 6 volts or 12 volts. You can recharge a 12 volt battery in all the conventional ways you'd recharge a car battery. For 6 volt batteries, place 2 in series.
You can also clean up the batteries and check electrolyte levels in the usual way.
Now, brace yourself: most likely, all 3 batteries are shot. Lead-acid batteries last only 5 to 10 years at most. So you'll want to positively identify the battery from whatever labeling may remain intact, and shop for replacements.
Here, be warned: having gotten a suprisingly favorable deal on many a machine, there is usually a ...reason... for that. Usually a lingering difficult-to-find glitch that makes the machine difficult to use, or a high-dollar repair which is unavoidable. Cost of battery replacement might explain it, but just the same, watch for -other- reasons the machine might be sidelined.
The batteries serve two purposes: their weight counter-balances the machine to make it stable, and of course electrical power. The batteries are large to give long run-time, not high amps. Simply for testing, car batteries of the same voltage will suffice, since car batteries have very high amps for their size. Beware that the machine will be tipsy with batteries missing or the wrong weight.