Final answer:
The worksheet reference in the formula is "Sheet3!" which indicates where the SUM function is being applied. The formula contains a typo, as the correct syntax should include a range of cells within the SUM function.
Step-by-step explanation:
The part of the formula "Sheet3!SUM A12 A6:A19" that references the worksheet is "Sheet3!". This indicates that the SUM function is being applied to cells on a worksheet named Sheet3 within the spreadsheet. Specifically, the formula is attempting to sum up the values from cell A12 through the range A6:A19 on Sheet3, although there is a typo in the formula since the correct syntax for the SUM function is =SUM(A6:A19) and doesn't usually include a single cell like A12 outside of the range.