Dehydration: This condition can cause a relative increase in RBC count due to decreased plasma volume, while the hemoglobin and hematocrit (h/h) levels remain normal. The low Mean Corpuscular Volume (MCV) could be indicative of microcytic anemia, but in the context of dehydration, it might reflect a concentration effect. The low platelet count could be due to dehydration-induced hemoconcentration or other secondary effects.

Chronic Disease (e.g., Chronic Kidney Disease): Certain chronic diseases can lead to a normocytic or microcytic anemia (low MCV) and may also affect platelet production or survival, resulting in thrombocytopenia (low platelet count). The high RBC count might not be directly related to the disease but could be a compensatory response or due to erythropoietin stimulation in conditions like chronic kidney disease.
Iron Deficiency Anemia: Although typically associated with low RBC counts, iron deficiency can sometimes present with a high RBC count if the body is compensating for the lack of iron by producing more, smaller red blood cells (reflected by the low MCV). The low platelet count could be related to the underlying cause of iron deficiency or a separate issue.
Thrombotic Microangiopathy (TMA): Conditions like TMA can cause thrombocytopenia and microangiopathic hemolytic anemia, which might present with a high RBC count due to compensatory mechanisms and a low MCV due to the fragmentation of red blood cells.

Polycythemia Vera (PV): A myeloproliferative neoplasm that can cause an increase in all three blood cell lines, but sometimes presents with isolated increases in RBC count. The low MCV and low platelet count are not typical, but PV can have variable presentations, and missing this diagnosis could have significant implications due to the risk of thrombosis and transformation to more aggressive diseases.
Myelodysplastic Syndromes (MDS): These disorders can affect the production of all blood cell lines, potentially leading to a high RBC count, low MCV (indicative of iron deficiency or dysplasia), and thrombocytopenia. MDS can progress to acute leukemia, making early diagnosis critical.

Primary Bone Marrow Disorders: Rare conditions affecting the bone marrow, such as myelofibrosis or other myeloproliferative neoplasms, could potentially lead to these laboratory findings, although they are less common and might present with additional symptoms or findings.
Congenital Disorders: Certain congenital disorders affecting blood cell production, such as congenital dyserythropoietic anemias, could present with abnormal RBC indices and thrombocytopenia, although these would typically be identified earlier in life.