TORONTO—Small brewers in Ontario are pushing for changes to one of the world's more unusual beer-selling environments.

About 80% of retail beer sales in Ontario occur in a chain of stores that's jointly owned by Canada's three largest brewers: Anheuser-Busch InBev NV, Molson Coors Brewing Co. and Sapporo Holdings Ltd. Between them, those brewing giants account for about 88% of Canada's beer sales.

Known...