Felizmente, há regras para isso na seção inalada poison :
A creature can attempt to hold its breath while inside the area to avoid inhaling the toxin. A creature holding its breath receives a 50% chance of not having to make a Fortitude save each round. See the rules for holding your breath and suffocation. If a creature is holding its breath and fails the constitution check to continue doing so, rather than suffocating it begins to breathe normally again (and is subject to the effects of the inhaled poison if still in the area).
Assim, por RAW, é a opção 3 (com a ressalva de que ainda há 50% de chance de ser afetado pelo veneno a cada rodada).