NASHVILLE — A man who spent 28 years on death row for killing three people while robbing a Nashville convenience store was executed early today after telling relatives who witnessed his death to “trust in the Lord.” Cecil C. Johnson Jr., 53, was pronounced dead at 1:34 a.m. at Riverbend Maximum Security Institution in Nashville. Those killed in the July 1980 shootings were the market owner’s 12-year-old son, Bobby Bell Jr., and two men sitting in a cab outside, Charles House, 35, and driver James Moore, 41.