What vitamin/nutritional deficiencies could cause a low wbc (3.0)? i am thin (bmi 19) so im wondering if this could be a cause?

I can think of none that are characterized by low WBC alone. I would repeat wbc and compare to last one if ever. If have been sick with virus may do this. Check with your team.

We need to know this is a new finding or a chronic condition because the approach is different. Black people can have low white cell count, a benign condition called benign ethnic neutropenia. Low white cell count (leukopenia) may be present in many diseases and this is not a chronic condition. For your query, deficiency in B12, folate and copper could also cause leukopenia.