Consider Giving it More Time
In my experience, it certainly can take a couple of months to really settle into a job. The work you're doing now may be giving you good background experience that will be required for the job you were hired for, or your supervisors may be starting you with slightly different work and evaluating your performance. I would give it a couple of months before escalating your concerns further.
If There's Still No Change
It's good that you've spoken to your supervisors this early. They should definitely be the first stop for something like this. If nothing changes in the near future, they should be the first ones you talk to then also.
If your company is large enough to have an HR department, you could also make a stop there and talk to them about it to.
No matter who you're speaking to about this, make sure you have very clear descriptions of the job you were expecting and of what the job currently entails. If you can find it, consider taking a copy of the original job posting with its description into these meetings so you can substantiate your claims.
Unfortunately, if neither of these outlets resolves your concerns, it may be time to start a new job search. I know that leaving a job after only a few months may reflect poorly, but in interviews you should be able to explain that the job wasn't what you expected without it being held against you. Just try to keep a positive spin on it. Emphasize good points of the job and lessons you learned from it. You can also use it as a way to start a discussion of exactly what you are looking for to try to prevent it from happening again.
Regarding the unfair work distribution, that is an entirely separate issue that should be handled separately. Try to leave that out of the conversations about the types of duties you're being assigned.