We have complete 4 weeks in the first 28 days of the month. We have 3 more days x = 29th day, y = 30th and z = 31st days of the month.
x, y, z can be any week day.
If x is known then y and z are consequently fixed. There are 7 possible permutations for x. Hence, total number of permutations/combinations / possibilities are : 7
Sat Sun Mon, Sun Mon Tue, Mon Tue Wed,
Tue Wed Thu, Wed Thu Fri, Thu Fri Sat, Fri Sat Sun
The desired outcomes/permutations are the first three in the above list.
Sat Sun Mon, Sun Mon Tue, Mon Tue Wed
probability = 3 / 7