Don't be overly upset if a he/she after sexing turns into a she/he, later on down the road. I've had this occur, as (if honest) most others have had too. By 3 mos, you should be pretty clear on the results.
And, side note: My does may play hard to get at mating time, and after arousal, mount the buck. Even though both sexes have been confirmed, this occasionally confuses the novice. It is normal, doesn't mean she's been sexed incorrectly, and some does will do this as part of their mating routine.