At 3 months your combs are going to give you a good clue. The males will be getting red, the females flesh colored still. Three rows of peas is pretty much male, a single row is female. You can also look at the coloring. the birds above are great examples of the difference between male and...
The two birds on the left are male, the three on the right are female.
Female, male, male
Two males on the left, three females on the right.
all the birds in pictures by themselves are male.