The grenade was hurled at a hardware store at Raha Chariali and three persons were injured, Nagaon district Deputy Commissioner Adil Khan told PTI.
The injured were identified as Ganesh Bhuyan, Padma Nath and Nurjamal.
They were admitted to BP Civil Hospital here, Khan said.
Padma Nath and Nurjamal are traders and Ganesh Bhuyan was a passer-by, Khan said.
Local militant outfit Tiwa Liberation Army has claimed responsibility for the blast.