Final answer:
The core principle in the three public sector personnel strategies is selection based on election, appointment, and rules.
Step-by-step explanation:
The core principle in the three public sector personnel strategies is selection based on election, appointment, and rules. In the competitive service, positions are filled based on qualifications such as work history, education, and civil service exam scores. Excepted service positions have their own hiring process, while senior executive service positions require experience in executive positions. The aim of these strategies is to ensure that the most qualified candidates are selected.