Three more suspects have been arrested in what police said appears to be a series of attempts by the sons of a Hollywood Boulevard restaurant owner to have their father killed. Derrick R. Plantt, 27, of Hollywood, Richard Gregg , 24, of Tulsa, Okla., and a 17-year-old girl are in custody in connection with an assault last July on Albert Junatanov, 45, in his room at Hollywood Presbyterian Hospital, where he was recovering from a stab wound in the stomach. Police said the girl, dressed as a nurse, allegedly injected Junatanov with a caustic substance meant to kill him. It didn’t. Sociz Junatanov, 18, and Asror Junatanov, 21, were arrested last month and charged with soliciting an undercover Los Angeles police officer to kill their father. In all, there have been at least six bungled attempts to have the elder Junatanov killed, detectives said.