My mom has low total wbc for 10 years. it's always around 3. though her absolutes counts of each type are normal. rbc and platelets normal. cause?

Likely no problem,,: The count may be low, but not very low and the absolute counts are normal. The absolute counts are more important as this means there's no problem with WBC production. It's also good that it's low as it likely means there's no infection or other illness present. If you're still concerned you can discuss this with your doctor, but it's highly unlikely there's anything. Best of luck.