Yeah, sorry, I wanted to keep it brief so I didn't really explain it properly.
"At least 3IV" is the same as "not 2IV", so I calculated it by doing the probability of finding a 2IV Pokemon and subtracting it from 1. The probability of finding a 2IV Pokemon is (31/32)^4 because in this case all of the four remaining IVs are non-perfect, and the probability of an IV being non-perfect is 31/32.
"At least 3IV" is the same as "not 2IV", so I calculated it by doing the probability of finding a 2IV Pokemon and subtracting it from 1. The probability of finding a 2IV Pokemon is (31/32)^4 because in this case all of the four remaining IVs are non-perfect, and the probability of an IV being non-perfect is 31/32.