The first incident took place at the G101 store on Kilbarchan Road in Johnstone, Renfrewshire, at 11.50am on Sunday.
Two female members of staff, aged 28 and 61, were threatened by a man with a weapon who made off with cash and cigarettes.
At 10.20am yesterday, the same store was targeted again by a man who threatened two female staff before leaving with a quantity of cash.
Meanwhile, a 35-year-old member of staff at the Express service station in Forest Road, Cumbernauld, North Lanarkshire, was threatened with what appeared to be a gun at 5.20am on Sunday.
The male thief demanded money from the male shopkeeper and left with a three-figure sum of cash as well as cigarettes.