The league's bottom side faced an uphill struggle to avoid the drop prior to the weekend's clash at the Gwangyang Stadium and made a dreadul start to the game, falling behind to a Stevica Ristic goal nine minutes in.
While Cho Dong-geon levelled before the half-hour mark, Leandro Lima restored the hosts' lead before the interval, with Ristic adding a third late on.
It leaves Sangju three points adrift of Seongnam, having played a game more, and with a vastly inferior goal difference of -25.
Requiring a win over Gyeongnan, two defeats for Seongnam as well as a 16-goal swing, the bottom side's fate is likely to be sealed on Wednesday when Kim Hak-beom's men play their game in hand against Incheon United.
Gyeongnam failed to get the win they needed to be certain of avoiding the relegation play-off, as they were beaten 1-0 by Busan I'Park - Brazilian striker Nilson on target.
The battle to avoid fifth place in the relegation round will go into the final week, with Gyeongnam and Seongnam still not safe, while at the top all that remains to be decided is who will secure the final AFC Champions League spot.
Jeju United remain in the hunt after a late Jin Dae-sung goal ensured them a 1-0 victory over with Ulsan, with Wednesday's clash between Pohang Steelers and Seoul likely to prove critical in determining the final qualifier.
Champions Jeonbuk Motors continued their dominant season by securing a ninth consecutive league win.
Motors were given a scare early in the second half when they conceded their first league goal since September, but maintained their run courtesy of late efforts from Lee Seung-hyun and Lee Sang-hyup.
