How would you journalize an adjusting entry if the entries are from June - August. Would you just x the insurance expense x 3 to account for 3 months?

If Aug 31 is the end of an accounting period and or are trying to do an adjusting entry for insurance used for a three month period then you are correct.

You would have to calculate the mothly amount of insurance and times it by 3 to get the total amount of insurance used.

The adjusting entry is:
Debit Insurance Expense for the amount of insurance used
Credit Prepaid Insurance for the amount insurance used