You can combine an OR statement with an ISBLANK statement and then wrap it all in an IF statement.
To begin, we need to check if a cell is blank, you can do this like so:
This will return FALSE if the cell is full and TRUE if the cell empty.
Next you want to check if all three cells are blank. This can be done with an OR statement, like so:
OR checks if all arguments are TRUE, and returns TRUE or FALSE. It returns FALSE if all arguments are FALSE, which is what you want.
Finally, you need to evaluate the statement and return your answer only if all cells have been filled in, otherwise the cell will remain blank. To do this, you use an IF statement, like so:
=IF(OR(ISBLANK(A3),ISBLANK(B3),ISBLANK(C3)), "", A3-B3-C3)