At least three soldiers were killed and four critically injured after a powerful blast hit an army convoy in Digboi in Tinsukia district of Assam, an army official said. Army sources say, heavily armed terrorist of banned outfit United Liberation Front of Asom (ULFA) attacked an army convoy early this morning with Rocket Propelled Grenades (RPG), Improvised Explosive Device (IED) and AK-47 hitting two trucks and a jeep near the Pengeri reserve forest in the eastern Assam district of Tinsukia, about 550 km from the state capital Guwahati.