Sonic (I don't know Sonic very well) practically only has close combat attacks, and could easily be hit with a flame from Spyro or fireball from (Fire)Mario. With enough rings he could last for a while(unless Spyro has Sparx with him to grab the rings before his nose), or he could just run away from the whole battle with his super speed.

That leaves Mario and Spyro, of whom I think Spyro would win because he doesn't need a powerup to do his (short)ranged flame attack.