The news follows the recent resignations of Europe vice-chairman Sally Ward and senior vice-chairman David Yelland (PRWeek, 4 Aug).
Roberts joined WS two years ago, after a stint at Fleishman-Hillard. Before that he spent 25 years in journalism, including stints at CNBC and the BBC. Roberts said he plans to set up his own broadcast PR agency.
Warder, who joined WS in 2000, is to join Edelman in October as director of its technology practice - he will also set up a science and emerging technologies department.
Fraser, who was a PR practitioner before moving into HR at Edelman, has worked at WS since 2003. She plans to work as an independent consultant or take a part-time job.
All told PRWeek that the timing of their exits was coincidental. WS declined to comment.