6^5 =7776 possible outcomes (order counts)
N,A and B represent different numbers that can be thrown
NNNNN 6 ways
NNNNA 6*5*5 =150ways
NNNAA 6*5*5C2=30*10 = 300 ways
NNNAB 6*5*4 *5C2= 120*10= 1200 ways
1200+300+150+6 = 1656 (order counts)
P(at least 3 the same) = 1656/7776 = 23/108
I do not guarantee my answers, especially not counting and probability answers.
This answer does seem higher than I would have expected.