Venomfang - 4 level 3 characters (TPK?)
I have simulated the battle with my players (Paladin, Barbarian, wizard and Rogue). I assume they all get hit with the breath weapon unless they win initiative (after all, Venomfang isn't stupid). After the breath attack, I try to play the survivors cleverly, as my players are generally experienced players and 2 of them are strategic thinkers (though 2 of them are not). In about 50% of the simulations, they all die. In the other 50%, they barely survive with Venomfang running away after taking half his HP in damage.
Was this encounter really meant for four, level 3 players? 3 out of my players have very high constitution (even the wizard), which is why the dragon breath doesn't take nearly all of them out in most simulations (usually it takes out 1 or 2 of them out). No player in my party has the hit points to take the full 42 points of poison damage.
Perhaps I should assume my players will be clever, but outsmarting a green dragon isn't a safe bet.