Sepsis: This condition is characterized by a systemic inflammatory response to an infection, which can cause elevated white blood cell (WBC) count, fever, and pain. The low platelet count (thrombocytopenia) can occur due to the consumption of platelets in the formation of blood clots or due to bone marrow suppression. Sepsis is a common and critical condition that matches the given symptoms closely.

Bacterial Endocarditis: This infection of the heart valves can lead to fever, pain (especially in the joints or muscles), elevated WBC count, and low platelet count due to immune complex deposition. It's a serious condition that could explain the combination of symptoms.
Disseminated Intravascular Coagulation (DIC): DIC can cause both thrombocytopenia and elevated WBC count as part of its complex pathophysiology involving both clotting and bleeding. Fever and pain can also be present, making it a plausible diagnosis.
Viral Infections (e.g., Dengue Fever): Certain viral infections can cause a significant elevation in WBC count, thrombocytopenia, fever, and pain. Dengue fever, for example, is known for causing severe thrombocytopenia and can present with these symptoms.

Meningitis: Although it primarily presents with symptoms like headache, stiff neck, and altered mental status, bacterial meningitis can also cause fever, elevated WBC count, and pain. Thrombocytopenia can occur due to the infection's effect on the bone marrow or as part of DIC. Missing this diagnosis can be fatal.
Neutropenic Fever: This condition occurs in patients with significantly low neutrophil counts, often due to chemotherapy or bone marrow failure. Despite the low neutrophil count, the total WBC count might be elevated due to an increase in other types of white blood cells. It's critical to identify and treat the underlying cause promptly.
Thrombotic Thrombocytopenic Purpura (TTP): TTP is a rare blood disorder characterized by the formation of blood clots in small blood vessels throughout the body. This condition can lead to thrombocytopenia, fever, and pain, among other symptoms. It's a medical emergency that requires immediate attention.

Hemophagocytic Lymphohistiocytosis (HLH): This is a rare disorder of the immune system where it produces too many activated immune cells called macrophages and lymphocytes. HLH can cause fever, elevated WBC count, thrombocytopenia, and pain due to the infiltration of these cells into various organs.
Lymphoma: Certain types of lymphoma can present with systemic symptoms like fever, pain, elevated WBC count, and thrombocytopenia. Although less common, lymphoma should be considered in the differential diagnosis, especially if other explanations are ruled out.
Leukemia: Acute leukemias can cause elevated WBC counts, thrombocytopenia, fever, and pain. The presentation can be similar to an infection, making it a critical diagnosis not to miss, although it is less common than infectious causes.