Actually Mephisto freed himself, he was just waiting for Diablo and Baal and controlling the Zakarumites from the temple, he seems to be less hands on then the other 2. He also sent Leoric for Diablo to use.
And I believe that Baal was always physically the strongest, his soul did crack the Soulstone.
"Mephisto finally managed to overcome the powers of the Soulstone that imprisoned him enough to influence and corrupt the local priesthood, known as the Zakarum. The Demon then had the priests work spells to shatter the Soulstone into seven shards, breaking the mystic seal and freeing the Lord of Hatred. Mephisto drove the six smaller shards into the left palms of six Zakarum archbishops who were then charged with bringing him nourishment in the form of human sacrifices while running their temple. The largest stone pierced the hand of the Que-Hegan, the highest divine authority of the Zakarum faith. It was this body that the Demon used as his manifestation on the mortal plane, warping the mortal’s form into an approximation of his true visage. For years the corrupted religious leaders of the Zakarum have kept Mephisto supplied with information and misguided followers to carry out his plans of conquest. He eventually met up with Diablo, and the two began to work to enslaving the human race. However, Mephisto was later killed by the same adventurers (or adventurer) that would go on to kill Diablo. Mephisto's Soulstone was destroyed in the Hellforge after his defeat, ensuring that he demon stayed dead.
Mephisto is considered the most intensely evil brother of the three Prime Evils (and is also the oldest). So overwhelming is his evil that his malevolent influence can bring the dead back to life; these zombified beasts are filled with such anger for the living that not even death can quench their desire for pain and suffering. Mephisto may also be viewed as the most intelligent and calculating. Over the years, after having ensorcelled the Zakarum, he used his newfound power to bring the lands of Sanctuary under his heel through the martial power of warped Paladins, and sent the Zakarum deep into Westmarch, where his inwitting servant, King Leoric, would take over Tristram."
- from http://diablo.wikia.com/
0
And I believe that Baal was always physically the strongest, his soul did crack the Soulstone.
"Mephisto finally managed to overcome the powers of the Soulstone that imprisoned him enough to influence and corrupt the local priesthood, known as the Zakarum. The Demon then had the priests work spells to shatter the Soulstone into seven shards, breaking the mystic seal and freeing the Lord of Hatred. Mephisto drove the six smaller shards into the left palms of six Zakarum archbishops who were then charged with bringing him nourishment in the form of human sacrifices while running their temple. The largest stone pierced the hand of the Que-Hegan, the highest divine authority of the Zakarum faith. It was this body that the Demon used as his manifestation on the mortal plane, warping the mortal’s form into an approximation of his true visage. For years the corrupted religious leaders of the Zakarum have kept Mephisto supplied with information and misguided followers to carry out his plans of conquest. He eventually met up with Diablo, and the two began to work to enslaving the human race. However, Mephisto was later killed by the same adventurers (or adventurer) that would go on to kill Diablo. Mephisto's Soulstone was destroyed in the Hellforge after his defeat, ensuring that he demon stayed dead.
Mephisto is considered the most intensely evil brother of the three Prime Evils (and is also the oldest). So overwhelming is his evil that his malevolent influence can bring the dead back to life; these zombified beasts are filled with such anger for the living that not even death can quench their desire for pain and suffering. Mephisto may also be viewed as the most intelligent and calculating. Over the years, after having ensorcelled the Zakarum, he used his newfound power to bring the lands of Sanctuary under his heel through the martial power of warped Paladins, and sent the Zakarum deep into Westmarch, where his inwitting servant, King Leoric, would take over Tristram."
- from http://diablo.wikia.com/