The only way to sex a Budgie visually is by the color of their cere, and their ceres won't be their permanent color until they around 1 year-old. They are born with either solid pink ceres or solid white ceres, and they change from there up until the age of a year old, so you can't tell their gender if they are truly under a year old... So at less than a year old you have no way at all to tell either of their genders. The lines black lines on their foreheads are an indication of age, the closer the lines are the cere the younger they are, and the more blank forehead with no lines they have the older they are...
Budgies don't go through their first molt until around a year old either, so if they are only 3 months or so, you have quite a while before either of them will molt or you will be able to tell their genders...They also will not be able to mate/breed until they are at least 6 months old, usually closer to a year but sometimes they do start younger than that, you just never know.
If you post a photo of both your Budgies we can estimate about how old each of them are, but if they are younger than a year old then we can't help you with telling their genders, as their ceres continue to change colors up to a year...Most Budgies are born with either all-pink ceres or all-white ceres, and they change from there. So the only way you can get them sexed for a while will be by having a feather DNA test done (they are too small for a blood-draw, unless they use a toenail, which can be inaccurate).