CBS is developing a third hour for its “This Morning” show, to be hosted by Gayle King. Sources say CBS is hatching a medically themed hour, like “The Doctors,” possibly titled “On Call.” A source adds that King “will co-host with Jonathan LaPook, the medical correspondent for ‘CBS Evening News With Scott Pelley.’ Gayle has done extraordinarily well on the morning show, along with Charlie Rose. [CBS] thinks she can easily extend her workload to [incorporate] a third hour . . . focusing on health issues. Gayle can tackle both hard news and lifestyle segments with ease.” A CBS rep commented: “CBS News regularly experiments on projects with existing talent, but we have no new programming plans.”