There is nothing wrong in drilling a tooth 3 times or multiple times unless it reaches to soft tissue zone (pulp horn). Drilling is done to shape the cavity and to remove any carious/ decayed part of tooth inside. Also thickness or length of enamel in any tooth depends on age of the individual, tobacco and other chewing habit etc. This also varies in a tooth itself. At the edge maximum and sides at the enamel- gum junction is least. Average length/ thickness of enamel in concerned tooth is 2- 2.8 mm. Do not worry, let your dentist
know your question and he will let you know. Only quakes will get offended.