Cheshire police said one of the gang members was 6ft 6ins tall and had a large build.
He was wearing a balaclava, a black beanie hat and a black North Face jacket.
His sledgehammer had a black head and brown handle.
The second suspect was slim and between 5ft 7-8ins tall.
He was wearing a black shellsuit and a peaked cap.
No description of the third gang member is available.
A Cheshire police spokesman said: “The shop assistant received minor injuries in the incident including soreness to the ribs.
“There were no other injuries.
“Anyone with information should contact Cheshire police on 101 or anonymously via Crimestoppers on 0800 555 111 quoting incident number 1206 of September 8, 2012.”
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