3yr old has mouth ulcers.low wbc@4.7 x1000mcl,neutrophils 38.5%(1.80x1000mcl), lymphocytes 50.1%(2.35x1000mcl)rest of blood work ok.what can be wrong?

Common issue: Mouth ulcers in a 3 yo are usually a viral process. They can be the initial infection of what would later become cold sores. They can be from a variety of gut viruses that just started with the oral cavity in this instance. The cited blood count is non-specific, and rather normal for age. Treatment is supportive until they self heal. The focus is on maintaining hydration.