3 mistakes in... >2 years? I feel like it's really hard to tell if the problem is really the person at that point. Have you had others perform the same job for a similar duration to see if they avoid the same mistakes?
If you made a list of every mistake each person makes in 2-3 years, and omitted all other detail, pretty much everybody would look like a terrible person. Context, frequency, etc. all matter.
If particular systems or people are seeing a high frequency of mistakes, maybe the system design is at fault, not just the person. Obviously it's hard to do in practice, but the ideal is to design systems that are mistake proof.
