The Edinburgh-based venue operator is one of only 11 companies in the corporate sector to achieve Gold status.
The company won the accolades for its cafés at the Modern Art Galleries, comprising of the Gallery of Modern Art, the Dean Gallery and the National Museum of Scotland.
It already holds a Gold award for its main head office operation at the capital's North Fort Street.
The Green Tourism scheme is the only certification scheme validated by Visit Britain through the International Centre for Responsible Tourism. Businesses opting to join are assessed by a qualified grading adviser against criteria covering a range of areas such as energy and water efficiency, waste-management and biodiversity.
Heritage Portfolio business development manager Kirsty Hepburn said: "Achieving these GTBS awards is an extremely significant step forward. In previous years, we were awarded three Silvers and one Bronze, so to win three Golds is testament to our commitment to sustainability and the environment, and innovation within what is a hugely important area of our business."