Note sure which day you are using, Wednesday or Thursday?
Anyway you can use the WEEKDAY function to return the day number:
=IF(WEEKDAY(B3)=2,F3-C3,IF(WEEKDAY(B3)=5,G3-C3,"") )
Where WEEKDAY returns one of the following:
Sunday 1
Monday 2
Tuesday 3
Wednesday 4
Thursday 5
Friday 6
Saturday 7