I have a 12 year old Honeywell Chromotherm III thermostat that is now showing repl batt. I replaced the 3 AA batteries w/new ones. The message was still there. I replaced them with another set. Still no change. I went through this process w/3 new sets of batteries from 2 different boxes. Am I dense or do I need a new thermostat??

You should replace it anyway now there is a chromotherm 8 I believe now. Try getting the honeywell pro 8000 with a backplate to fit that. Or a new t stat good luck. It's the stat you need to buy another.

I have a 12 year old Honeywell Chromotherm III thermostat that is now showing repl batt. I replaced the 3 AA batteries w/new ones. The message was still there. I replaced them with another set. Still no change. I went through this process w/3 new sets of batteries from 2 different boxes. Am I dense or do I need a new thermostat??

This and other display problems happen on the old style III Honeywell. I had mine replaced years ago under warranty. Honeywell knows about the problem but I do not believe you will get a warranty replacement because of the age of your t stat