The three stores Â two former GP Stores in Torquay and one store in Plymouth Â are situated in popular tourist areas. All staff from the new stores will transfer their employment to the society.
Plymco chief executive Douglas Fletcher said Plymco wanted to become the leading convenience retailer in the south West.
He said: "These three new food stores are timely acquisitions, enabling the society to continue its expansion of neighbourhood c-stores."
Plymco has also bought a site near its Modbury store, which it plans to develop as housing and a larger food store, closing its existing premises.
At the same time the Society is developing a major new 20,000sq ft non-food outlet located in Launceston, part of an Â£8m retail park development,
In addition to the Co-op's Homemaker unit, the site will feature a DIY store and a range of other non-food outlets.