The worker was threatened by a man who stole money from the till and smashed the victim's phone at the Whetstone Green store on Fordhouse Road.
The offender fled after the attack, which took place on January 3 at 7pm.
Minutes later two women then entered the shop and took the victim's phone before leaving.
Now CCTV images of the three suspects have been released as part of a police appeal.
Anyone with information is asked to contact the force via Live Chat at west-midlands.police.uk between 8am and midnight, or by calling 101 anytime quoting crime reference number 20WV/2601D/20.
Alternatively, Crimestoppers can be contacted anonymously on 0800 555 111.