Viral Gastroenteritis: This condition is often associated with headache, nausea, and fatigue due to the body's response to the viral infection. The symptoms can be nonspecific but are commonly seen together in gastrointestinal infections.

Migraine: Migraines can cause severe headaches, nausea, and fatigue. They are relatively common and can be triggered by various factors, including stress, certain foods, and hormonal changes.
Tension Headache: Tension headaches are another common cause of headache and can be accompanied by nausea and fatigue, especially if the headache is severe or persistent.
Influenza: The flu can cause a wide range of symptoms, including headache, nausea, fatigue, and body aches, making it a likely diagnosis during flu season or in the context of a flu outbreak.

Meningitis: Although less common, meningitis is a serious and potentially life-threatening condition that can present with headache, nausea, and fatigue, along with other symptoms like fever and stiff neck. Early diagnosis and treatment are crucial.
Subarachnoid Hemorrhage: A sudden, severe headache (often described as "the worst headache of my life") accompanied by nausea and fatigue could indicate a subarachnoid hemorrhage, which is a medical emergency.
Carbon Monoxide Poisoning: This condition can cause nonspecific symptoms like headache, nausea, and fatigue, especially in the absence of a clear infectious or traumatic cause. It's critical to consider environmental exposures.

Brain Tumor: While rare, brain tumors can cause a variety of symptoms, including headache, nausea, and fatigue, depending on their location and size. These symptoms might be persistent or progressively worsen over time.
Chronic Fatigue Syndrome: This is a condition characterized by extreme fatigue that cannot be explained by any underlying medical condition. It might also include symptoms like headache and nausea, although the primary complaint is usually the profound fatigue.
Mitochondrial Disorders: These are a group of disorders caused by dysfunctional mitochondria, affecting almost any organ system. They can present with a wide range of symptoms, including headache, nausea, and fatigue, often in combination with other systemic symptoms.