Three Massachusetts health-care systems combined their assets to form Southcoast Health System in June 1996 to compete against providers north in Boston and west in Providence, Rhode Island. Charlton Health System, St. Luke's Health System and Tobey Health Systems totaled $319 million in 1995 revenue. Southcoast has 13 subsidiaries offering health services, including one unit operating three hospitals. The merger comes as Columbia/HCA Healthcare Corp. explores a deal with Roger Williams Medical Center in Providence.