I think 3 months is long enough to give a medication a fair trial. I would ask for a different med. Supposedly different people react differently to different meds.
Mood stabilizers are sometimes added when a person's depression is not responding to other meds. Small doses of antipsychotics are also used to "boost" antidepressants. But I would think a dr would try a different antidepressant before a mood stabilizer or antipsychotic.