The unnamed man was at Hunslet Carr Sports and Social Club in the south of the city when the gang burst in to the building demanding money.
West Yorkshire Police said the man was hit with the hammer before the three raiders fled with cash.
He was taken to Leeds General Infirmary with a head injury.
Officers said the raiders escaped in a stolen blue VW Passatt which was later found abandoned on Telford Terrace in Hunslet.
The thieves are all described as white, of medium build and wore balaclavas. One wore tracksuit bottoms and another a light blue jumper.
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