A: At first glance, this does look tricky, but after some thought, and close reading of the cards, the answer is pretty clear.

Out of Ammo (OoA) causes the Can of Gasoline and the Chainsaw to be discarded… unless one person has both, in which case the Chainsaw can be kept: it runs out of fuel, but is refilled by the Can of Gas. The Flamethrower specifies that it, too, is discarded if it runs Out of Ammo.

The only thing that seems confusing is that the Flamethrower ALSO specifies that it “counts as the Can of Gasoline.”

So… the Chainsaw can be “saved” from being Out of Ammo if you have either the Can of Gasoline or the Flamethrower. Nothing can “save” the Flamethrower or the Can of Gasoline, however, so if you have all three when Out of Ammo is played, all you’re left with is the Chainsaw.

We could also look at it as three possible sets of pairs:

(Gasoline + Chainsaw) OoA = Chainsaw
(Flamethrower + Chainsaw) OoA = Chainsaw
(Gasoline + Flamethrower) OoA = nothing

Although, thematically, it might seem that the Flamethrower should be equivalent to the Chainsaw in that it seems like it should be able to be “refilled” (“saved”) by the Can of Gas… well, we didn’t think of that, so it’s not specified, so that’s not how it works. There probably wouldn’t have been enough room on the card to describe that anyhow.

UNOFFICIAL POSSIBLE HOUSE RULE:
If you wanted to play that as a house rule, or course, that would be fine with us. If that were the case, then when you had all three when you run Out of Ammo, you’d be able to keep either the Chainsaw or the Flamethrower to be “saved” by refilling with the Gasoline, your choice. That is not an official ruling according to the cards, however. Regarding the pairs above, of course, it would look like:

(Gasoline + Chainsaw) OoA = Chainsaw
(Flamethrower + Chainsaw) OoA = Chainsaw
(Gasoline + Flamethrower) OoA = Flamethrower