The three sites are located in Knebworth, Hitchin and Welwyn Garden City. The Hitchin site also represents Alfa Romeo from a separate showroom.
Both the Hitchin and Knebworth sites have Esso filling stations.
All of the Lisles staff will be transferred to Marshall.
Roger Knight, Marshall chief executive, said: “As two private family-owned businesses, there are many similarities between Marshall and Lisles. We expect the integration process to be simple and quick to complete.”