Can a plaintiff in a common-law negligence action against three other parties claim damages for common law negligence?

This proceeding involves the application of the principle of Witt v. Jackson, 57 Cal.2d 57, 17 Cal.Rptr. 369, [213 Cal.App.2d 242] 366 P.2d 641, to a personal injury action which was at issue before that case was decided. A plaintiff, who has received compensation from his employer, has brought a common-law negligence action against three other parties. These defendants now desire to plead that the employer was also negligent, attempting thereby to bar such portion of the plaintiff's recovery as would be applied to the benefit of the employer.