C'mon guys, front rings do wear out. Wear there isn't easy to detect visually, either.

Divinitous, is it skipping in all front gears? If so it isn't a front wear problem.

The middle gear on a triple is usually first on a road bike, but sometimes a heavily-used granny goes too. (Fewer teeth means more wear per tooth, but some gears are used more frequently than others.)

There are definite differences in front chainrings. I have an old thread here where I measured several models of road cranksets and few were identical. Some were radically different. However, between versions of the same model - like M751 and M752 - they should be interchangeable. Did you look at the interchangeability chart on the Shimano exploded drawings? So long as you stick with 32T, the middle rings are 'A' rated for interchangeability.

One can't make blanket statements about the granny. Two of the variables are plate thickness and tooth offset (from the spider mounting surface). Also ring bolt countersink. Unless you know what you're doing, best to stick with the spec'd parts.