Colic: This is the most common cause of inconsolable crying in infants, especially at this age. It is characterized by episodes of crying lasting more than 3 hours a day, more than 3 days a week, in an otherwise healthy infant. The exact cause of colic is unknown, but it is thought to be related to gas, overstimulation, or sensitivity to milk proteins.

Gastroesophageal Reflux Disease (GERD): Infants with GERD may cry due to discomfort or pain from stomach acid flowing back up into the esophagus. This condition is common in infants and can be managed with positional changes and, in some cases, medication.
Gas and Bowel Movements: Gas can cause significant discomfort in infants, leading to crying. Similarly, constipation or difficulty passing bowel movements can also cause distress.
Overstimulation or Understimulation: Infants can become overwhelmed by their environment, leading to crying. Conversely, a lack of stimulation can also cause fussiness.
Tiredness or Overfatigue: Infants, especially at 3 weeks, have limited ways to communicate their needs. Tiredness can manifest as crying if the infant is having trouble settling down or is overtired.

Infection (e.g., Urinary Tract Infection, Meningitis): Although less common, infections can cause severe irritability and crying in infants. It's crucial to consider and rule out infections, especially if there are other symptoms like fever, lethargy, or feeding difficulties.
Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood flow. It's a medical emergency and can present with sudden onset of severe crying, especially if accompanied by vomiting, bloody stools, or a palpable abdominal mass.
Fracture or Trauma: Accidental or non-accidental trauma can cause an infant to cry inconsolably. Any suspicion of trauma warrants immediate investigation.
Congenital Issues (e.g., Congenital Heart Disease, Hirschsprung’s Disease): Certain congenital conditions can cause discomfort or distress leading to crying. While these conditions are typically identified at birth or shortly after, some may not be immediately apparent.

Food Allergy or Intolerance: Though less common, an allergy or intolerance to a component of the infant's diet (most commonly cow's milk protein) can cause significant distress and crying.
Neurological Conditions: Rare conditions such as hydrocephalus, subdural hematoma, or other neurological anomalies can cause irritability and crying in infants.
Metabolic Disorders: Certain metabolic disorders can present with nonspecific symptoms including irritability and crying. These are rare but critical to diagnose early for effective management.