Final answer:
The average income in the county is the most likely third variable to be considered.
Step-by-step explanation:
A third variable that is likely to be considered in this scenario is the average income in the county. The number of books sold within a county may be influenced by the average income level, as higher-income individuals may have more disposable income to spend on books. Similarly, the number of burglaries committed may also be influenced by the average income, as areas with higher average incomes may attract more burglars.