The men, who were all wearing dark clothing, entered the post office in Greenside shortly before 4.50pm on Tuesday, December 11.
They staff inside with what is believed to have been a bladed weapon, then escaped with a quantity of cash.
Now Kirklees District CID has issued an appeal for witnesses and information.
Detective Inspector George Bardell, of Kirklees District CID, said: “Detectives are continuing to investigate what was clearly a frightening incident for staff and we would like to speak with anyone who saw the robbery.
“We also like to speak with anyone who saw the van fleeing the scene in the aftermath or has any information which could assist detectives."
All of the suspects were white, with one of them described as thin, between 5ft 7in and 5ft 10in. He wore a black balaclava, black top, black gloves and grey jogging bottoms.
Another was also wearing a black balaclava, a dark jacket with a grey hoodie underneath, dark jogging bottoms, light green gloves and dark trainers.
The third was wearing a black balaclava, grey jacket, dark blue jogging bottoms and gloves.
Police said no-one was hurt during the incident.
Anyone who has information is asked to contact Kirklees CID on 101, referencing crime number 13180620685.
Information can also be given anonymously to the independent Crimestoppers charity on 0800 555 111.