OK: returned from the parallel universe, and understand your question.
You're seeing it from a project management point of view, such that
workdays in between the start and stop date add 9 hours to the total:
got it. Try this formula:
=(17/24-MOD(A3,1))+(MOD(B3,1)-8/24)+((NETWORKDAYS(A3,B3)-2)*9/24)

This formula determines the number of hours worked from the start time
until 5pm on the first day, adds it to the number of hours between 8 am
and the end time, and then determines the number of working days and
adds 9 hours for each day in between. Note you can add holidays to the
NETWORKDAYS function to disregard known days off.