Raiders held up a garage worker at knifepoint before ordering him to open the safe.
The three men escaped with a considerable amount of cash, cigarettes and phone cards from a Total garage.
They struck on Sunday at 9pm while the worker was alone inside the garage on the A272 Haywards Heath Road, Piltdown, near Uckfield.
Police today appealed for help. They especially want to speak to a woman who may have seen the suspects as she filled up her car with petrol.
A force spokesman said: "This woman is not a suspect but we believe she may have valuable information about the suspects.
"We appeal to her to call us as soon as possible as her information may be very important to our inquiry."
A group of men were also seen in the garage's car park at about 9pm. They are believed to have been in a dark-coloured hatchback.
Anyone with information can call police on 0845 6070999 or Crimestoppers on 0800 555111.
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