The thefts took place within a 30-mile radius in Inverclyde and Renfrewshire early yesterday.
A small silver hatchback car was spotted at each scene and the description of the suspects are similar, Strathclyde Police said.
The first raid happened at about 6.45am when two men armed with a knife entered Ashton Stores in Gourock and escaped with cash.
A man entered Spar in Wemyss Bay at 7.30am and threatened staff before stealing cigarettes.
In the third incident, at 9.30am, two men entered Howwood General Stores in Howwood and threatened a woman with a knife before stealing money.