If it has no bristles it is a female. As far as I know that is the only way to know for sure.
At 3 inches with no bristles I would say female 99% .
As others have said, 3 inches should be big enough to assume you have a female. Although I have had males that develop later too, at 9 months or so (size variable) especially if there are any other plecs in the tank. Subdominant males develop a lot slower and may not show bristles for up to a year, especially if there is a dominant male in the tank.
Females will fill up with eggs and get a nice round belly though, so thats another good indicator too!
i agree with everyone else and I have been told that you can also tell by their shape, yours is slightly rounder between the 2 sets of fins rather than very slimline which is another sign of a female, but much less reliable than the bristles like everyone else has said
does she have a name..?
A lot easier to sex if a picture of the OTHER side is shown....