What is the difference between cold, flu and catarrh?

Depends. Depends on what you mean by the words. A cold can be caused by many different kinds of viruses. Influenza which some call flu is a particular virus. Sometimes when people vomit others will say they have the flu. "catarrh" is a cultural word that means a runny nose that is usually associated with a cold.
Viral infections. A cold is a viral infection involving the nasal mucosa (lining). It may sometimes develop into a sinusitis. It is a self limited illness and is super-infected with bacteria in about 1%-2% of cases. Flu is a viral infection which is more systemic and involves the whole body with multiple symptoms of fever and generalized aches. Catarrh is mucus secretion from the nose and usually is viral.