Was not quite certain of the requirement (but seem to have hit upon an acceptable solution!):
Use =NETWORKDAYS(A2,B2)>2
under Format values where this formula is true: under Use a formula to determine which cells to format in Conditional Formatting.
Returns the number of whole working days between start_date and end_date. Working days exclude weekends and any dates identified in holidays.
The syntax is:
NETWORKDAYS(start_date,end_date,holidays)
Dates should be entered by using the DATE function, or as results of other formulas or functions.