*what happens when a troll hits 0 hp from subdual?
Strictly speaking subdual damage (now called nonlethal damage in 3.5e) does not hits 0 hp. You must realize that nonlethal damage does not reduce hp, but the total nonlethal damage is recorded separately (a positive number). And when the total of nonlethal damage = his current hp, he is staggered. When the total of nonlethal damage exceeds his current hp, he is unconscious. Read P.146 of PHB for more detail.
*How long is it unconscious?
Until his total nonlethal damage is reduced (or his current hp increased) and that is no more greater than his current hp. Or, he has a 10% chance every minutes to wake up. Read P.314 of PHB for more detail.
*What effect does his 3 hp regeneration have on the subdual damage
Regeneration ability heals nonlethal damage (only). So the troll heals regenerate 5 (not 3, actually) nonlethal damage per round.
*What would a cure wounds do in this instance?
Something heals hp also heals the same amount of nonlethal damage. So, if a Cure Light Wounds spell heals 5 hp, now his current hp is cured by 5hp, and also, his nonlethal damage is reduced by 5.
*Once unconscious how many hp of damage from a fire or acid source are needed to kill him?
Unconscious or not, he dies when his hp is reduced to -10. Or, he can be killed by coup de grace and such.