Growing Pains: This is a common condition in children around the age of 3, characterized by recurrent leg pain, typically in the evenings or at night, without any signs of inflammation or trauma. The pain is often bilateral and can interfere with sleep.

Viral Myalgia: Various viral infections can cause muscle pain in children, including the legs. The absence of swelling or bruising and the presence of pain without other specific symptoms could suggest a viral etiology.
Overuse Injury: Although less common in a 3-year-old, overuse injuries can occur in children who are very active. However, this would typically be associated with some history of increased physical activity or trauma.
Nutritional Deficiencies: Deficiencies in vitamin D or calcium can lead to muscle pain and weakness. This is more likely in children with limited sun exposure, dietary restrictions, or malabsorption.

Child Abuse: It's crucial to consider non-accidental trauma, especially if the pain is persistent, and there are inconsistencies in the history provided by caregivers. However, the absence of bruising or swelling makes this less likely but not impossible.
Leukemia or Other Malignancies: Bone pain at night can be a presenting symptom of leukemia or other bone malignancies. Although rare, these conditions are critical to diagnose early.
Osteomyelitis or Septic Arthritis: Infections of the bone or joint can cause severe pain and are medical emergencies. The absence of fever, swelling, or redness does not rule out these conditions entirely.
Neurological Conditions: Certain neurological conditions, such as peripheral neuropathy or spinal cord tumors, can cause leg pain. These are rare but important to consider if other symptoms are present.

Juvenile Idiopathic Arthritis (JIA): This chronic autoimmune disease can cause joint pain and swelling. It's less common in children as young as 3 but should be considered if there's morning stiffness or swelling.
Fibromyalgia: Although more commonly diagnosed in adults, fibromyalgia can occur in children, characterized by widespread musculoskeletal pain accompanied by fatigue, sleep, and mood issues.
Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Defects in connective tissues cause the signs and symptoms of these conditions, which can include joint pain.

Detailed history of the pain: timing, location, severity, and any relieving or exacerbating factors.
Dietary history to assess for potential nutritional deficiencies.
Developmental and medical history to identify any predisposing conditions.
Physical examination to look for signs of trauma, infection, or neurological deficits.
Laboratory tests may include:
- Complete Blood Count (CBC) to rule out leukemia or infection.
- Erythrocyte Sedimentation Rate (ESR) or C-reactive protein (CRP) to assess for inflammation.
- Vitamin D and calcium levels to evaluate for nutritional deficiencies.
- Imaging studies (X-rays, MRI) if there's suspicion of trauma, infection, or malignancy based on history and physical examination findings.