Police said a local journalist had ordered the mugging.
Police seized a toy pistol from them.
The BCL activists include former district unit publicity secretary Abdur Rahman, 28. The other two – Saiful Islam Rana, 23, and Tokon alias Kopa Tokon, 24 – are activists of the Awami League student front.
Witnesses said the incident took place on Thursday night at Damurhuda upazila's Koshaghata.
Flour trader Shahin, a Koshaghata resident, was returning home after collecting dues when the BCL activists blocked the road with two motorcycles.
Locals rushed to his aid when he shouted for help as the muggers were fleeing.
The trader has filed a case.
Chuadanga's acting police Superintendent Belayet Hossain said local 'Doinik Akash' newspaper's acting editor Tanjir Ahmed Rony was behind the incident.
“Rony ordered the three to mug Shahin,” he said.
The journalist could not be contacted for comments.
Damurhuda police OC Abu Jihad Md Fakhrul Alam Khan said the three men had criminal records.