Without seeing your actual numbers it is not possible to give you an exact answer, however, the most likely reason you would not see an increase in refund when adding a third W-2 is because:
- additional income increases your tax liability,
- or you did not have any taxes withheld,
- or the amount of taxes you had withheld covered your increase in liability perfectly
The most important thing is to double check your numbers. If your numbers are accurate, then there is nothing to correct.