The heat in the garage will make them work harder, but shouldn't burn em out. You can help them out by placing foam around the freezer, the pink stuff they sell in sheets at home depot work well.

Check the coils and see if they are dirty (especially the ones that cool the freon down, they are the ones that get hot). That is the #1 reason most go out. Keeping the components on the outside dust free can revive most old fridges and freezers. On many fridges, the pipes to clean are the ones exposed on the back, some are a bit more difficult to get to, and could be in many different places, mine is underneath, with a cover over it.

Have any of them been on their sides or backs recently? If so, oil may have gotten into the system from the compressor. That can often be fixed by allowing it to not run at least as long as it was on it's side or back. Double that amount of time if it was running.

For all three to go bad so closely together, I would think that something they all have in common did it. Not sure what it might be since they are in different garages. The heat just doesn't seem to be the cause to me.... if it were, AC units would always be going out.