What could cause abnormally low levels of rbc, wbc, and platelets? Aplastic anemia, mds, leukemia have been ruled out via bm biopsy.

Infections-- but--- Much more detail is needed about your case. What's the age of the patient, comorbid states, medications, family history, environmental exposures, etc. This question is too important to have it answered correctly online- discuss it with your doctor who has all the facts in hand.
Toxic Chemicals. Inhaling fumes perhaps occupationally or recreationally could do it. Exposure to toxic chemicals. Your hematologist will be best able to guide you. Sometimes though these things just happen for no apparent reason.
Unknown. If you have already received a bone marrow biopsy under the direction of heme-onc, then you're probably beyond the scope of anyone here without your specific info. If your doc can't solve it then he/she may refer you to an academic center. Because so many things cause pancytopenia, it may take a while to find the offending med or disease. Best of luck!