93,000 miles, just had spark plgs rplcd, third coil misfire, suspicious?

Impossible to know what's happening here without diagnosing the vehicle, or at least knowing how the mechanic is justifying these repairs. If you could post more info, like what diagnostic trouble codes were stored and what parts were replaced during each repair, that would help. At this point, I would replace all the coils or have another shop look at this for another opinion.