Just to be clear, since your question states that the Pokemon is already Poisoned (and I am assuming it was not previiously Poisoned by Tri-Poison) and then the flip for Tri-Poison fails, you would only place 1 Damage counter for the Poison. Tri-Poison only places 3 for Poison when you flip heads. The fact that the Pokemon you are targeting is already Poisoned makes no difference.
BDS