City police arrested three persons on the charge of demanding money at knife point from one R. Angala Easwaran at Mani Mandapam Salai here on Monday. The accused – Sathaiyan, G. Raman and N. Ramanathan _ demanded Rs. 500 from Angala Easwaran by brandishing a knife and threatening the latter, the police said. The trio were later sent for remand. The Gandhi Market police has registered a case, sources said.