This question is based on conditional probability..
We know :
P(A | B) = P(A ∩ B) / P(B)
So here P(B) = P(maximum of 3 numbers = 6)
So we have 5 numbers to chosse from to choose remaining 2 numbers which can be done in 5C2 = 10 ways..
Hence P(B) = 10 / 8C3
Now coming to P(A ∩ B) , it suggests minimum of 3 numbers should be 3 and maximum should be 6..
So only possibility is to choose the middle number which can be 4 or 5 and hence 2 ways.
Hence P(A | B) = ( 2/8C3 ) / (10 / 8C3)
= 2 / 10 = 1 / 5
Hence required conditional probability = 1 / 5