The incident happened at the One Stop Shop in Usk at about 06:00 GMT.
A 15-year-old, from Little Mill, is currently in police custody after being arrested on suspicion of robbery.
Gwent Police said a 36-year-old man from Blaina and a 42-year-old from Little Mill had been arrested and released on police bail pending further enquiries.
Police investigating an armed robbery at a shop in Monmouthshire on Sunday have arrested a teenager and two men.