The market value of equipment had decreased by 30% of its original cost. Depreciation for the quarter totaled 1,000.00. (The equipment acct has 15,000.00 in it)

What would be the adjusting entry on September 30?

The market value has no bearing on this problem.

the journal entry will be:
Debit depreciation Expense for the amount given
Credit Accumulated Depreciation - Equipment Account for the amount given.

I am sort of confused as to what that amount would be. It states that depcreciation for the quarter is 1000.00. But I wouldn't make an adjusting entry at the end of September, for the entire 1000.00. So would it be for a third of that amount, meaning just 333.00 for the adjusting entry for end of September?

Being in your problem it states "depreciation for the quarter totaled 1,000.00", then the amount should be 1,000.00 because you are recording the depreciation for July, Augest, and September.