If they're that hot, I would think that the charging circuit is overcharging them. Before spending the money on new batteries, I'd measure the voltage presented by the charging circuit while the batteries are connected, then disconnect the batteries and measure them each individually.
If the charger was giving them more than a float voltage (27.8, within about .1), but both batteries were at (or above) 13.9 individually, then the charging circuit has either lost regulation on the float-charging stage, or is failing to switch from a bulk to a float stage.
On the other hand, if one (or both) of the batteries measure significantly below 13.9, you have one or more shorted cells. If the charger was still presenting the 27.8-ish volts to them, then it hasn't recognized the shorted cells. The charger *might* be bad, or it may have just been battery failure.
Those voltages are for dual 12V batteries in series. Half them if your unit uses 6V batteries.
As for a 3-year rule on SLAs, if they're properly cared for, that's the
*minimum* you should get. We replace our batteries at 3 years just as a precaution, but I have yet to see one fail before that time - *if* it's been properly cared for. Occasionally, someone will unplug their UPS, ignore the beeping until the battery is completely dead, and let the UPS sit like that for a month or two. That's not the battery's fault, it's the user's fault. The batteries that have failed on us have all been in units that were 4+ years old, often 5-6 years.