This is a highly debatable question. I have been playing tekken 3 for past 18+ years. I will be giving you my opinion.
In the king of iron Fist Tournament all players (characters) are equally powerful.
The king has the maximum number of moves inside tekken 3 and Jin has the maximum number of Combos.
Nina is the fastest of all.
Paul has the best moves that can be done quickly including the Burning Fist that takes very less time to actuate. Paul moves also take a lot of energy of the opponent in very less time.
King has the maximum number of moves that includes S.T.F , Rolling Death Cradle, Scorpion Death lock, Power bomb, Black bomb, Tombstone Pile driver etc. The most complex moves belong to King ranging from variety of combos to the chain grapples.
Law is also one of the most agile players in Tekken 3. His chain moves take a lot of energy of the opponent in a single chain hit.
Jin has the maximum number of combos and also has chain hits that take upto 60-80% of the energy of the opponent in a single chain hit.
It generally depends on your abilities how to use a player to the maximum potential . The more you play the more better you will perform in all circumstances.
Playing Tekken 3 is a lot about patience and performing the required moves under circumstances best suited to it. With any player you can always perform to the best of your potential.