If he changes his type and runs into resistance or weakness damage, then you apply it to the total damage, not each individual attack.
If Kecleon was a Water-type and used Triple Smash on a Grass-type card (Like Weepinbell from R/S Sapphire set, for example), and then landed two heads out of three, resulting in what would normally be 50 damage total, the resistance (-30 Water) would knock down Kecleon's attack to only 20.
Sorry about leaving that part out. It's a known rule in the TCG to account resistances/weaknesses after damage has been calculated, so I left out general info like that. I'll be more perceptive from now on. :)