Ryuta Koike, Aleksandar Cavric and Matheus Bueno netted in the final 21 minutes for Toru Oniki's side, who joined fellow J-League side Vissel Kobe on nine points from three matches.
"Our most important goal as a team was to make it three consecutive wins in the Asian Champions League," Koike said.
"Recently, we haven’t been able to win matches at home, and things haven't been going well in the league."
Koike broke Pohang's resistance in the 69th minute, redirecting Buenos' centre past goalkeeper Hwang In-jae with an improvised finish off his chest.
Cavric doubled the lead when he scored at the second attempt having seen his initial shot hit the post and Bueno completed the scoring from close range two minutes from time.
Kashima and Kobe are the only teams with perfect records remaining in the eastern side of the draw after Gamba Osaka were held to a 1-1 draw by Thailand's Buriram United.
The visitors took the lead when Eduardo Mancha netted just before the interval and Gamba's Takashi Usami equalised after 66 minutes.
"I'm proud of my team, to come to Japan and perform like that was impressive," said Buriram captain Kenny Dougall.
"When you take the lead and don't win it's always a bit disappointing but we have another game next week."
The top eight finishers in the 16-team leagues in both east and west Asia will advance to the last 16 in March while the quarter-finals, semi-final and final will be played on a centralised basis in Saudi Arabia in April and May.