What is the explanation for King's ending movie on Tekken 3?

I assume you mean this one: http://tekken.wikia.com/wiki/File:Tekken_3_-_King_ending_-_HD_720p

The King in Tekken 3 is not the original King. He apparently was killed by Ogre.
As a result, Armor King trained a replacement and entered him into the King of Iron Fist Tournament.

The ending doesn;t really explain it brilliantly, but depicts King II reaching the top levels and winning a wrestling championship. The part thats not explained great is that on realizing his mentor is not at ringside, he rushes to the locker room where he finds Armor King exhautsed, as he has been fighting a heart condition.

Tekken 3 King Ending