Viral upper respiratory tract infection (URTI): This is the most common cause of coryza (common cold symptoms) and hoarseness in children. The addition of diarrhea could be due to a viral gastroenteritis, which often co-exists with URTI symptoms in children.

Influenza: Influenza can cause similar symptoms, including coryza, hoarseness, and gastrointestinal symptoms like diarrhea, especially in children.
Adenovirus infection: Adenoviruses can cause a range of symptoms including respiratory infections (leading to hoarseness and coryza), gastroenteritis (diarrhea), and conjunctivitis.
Respiratory Syncytial Virus (RSV) infection: Although more common in younger children, RSV can cause upper respiratory symptoms and, less commonly, gastrointestinal symptoms.

Epiglottitis: Although rare, epiglottitis is a life-threatening condition that can present with hoarseness, difficulty swallowing, and respiratory distress. It's crucial to consider this diagnosis, especially if the child shows signs of severe respiratory distress.
Bacterial tracheitis: This condition can cause hoarseness, coryza, and significant respiratory distress. It's a medical emergency that requires prompt diagnosis and treatment.
Pertussis (Whooping Cough): Pertussis can cause a persistent cough, which may lead to hoarseness, and is accompanied by coryza and sometimes gastrointestinal symptoms. It's particularly dangerous in infants but can affect children of all ages.

Diphtheria: A rare but serious bacterial infection that can cause hoarseness and coryza due to a membrane forming over the tonsils and throat. It's more common in areas with low vaccination rates.
Mycoplasma pneumoniae infection: Typically causes mild symptoms but can lead to more severe respiratory infections. It's less common in young children but should be considered in school-aged children with persistent respiratory symptoms.
Yersinia enterocolitica infection: A rare cause of gastroenteritis that can also lead to respiratory symptoms, though this is less common.