When writing a function in Microsoft Excel, using A3 or $A$3 can make a big difference in how the function behaves.

A3 refers to the cell in the third row and the first column of the current worksheet. If you copy and paste a formula containing A3 to another cell, the reference will change based on the relative position of the new cell. For example, if you copy a formula containing A3 from cell B3 to cell C4, the reference will change to B4.

On the other hand, $A$3 is an absolute reference to cell A3. When you copy and paste a formula containing $A$3 to another cell, the reference will remain the same. For example, if you copy a formula containing $A$3 from cell B3 to cell C4, the reference will still be $A$3.

In summary, using A3 in a function makes the reference relative to the current cell, while using $A$3 makes the reference absolute. This can be useful when you want to refer to a specific cell or range of cells that should not change when you copy the formula to other cells.