If a 3 year old has low absolute monocytes (0.3) and low neutrophil % (28.3) what does it mean? Do I need a follow up?

It depends. It depends on the clinical picture and why the test was done. For monocytes usually, one low count is not medically significant. Same for neutrophils. What does matter is the pattern and direction of the trend. So yes check in with you child's pediatrician to assess the significance of these results for your child.