Enemy has deployed 3 bombs from ziggs on one battlefield. There are no units at this battlefield. I play salvage. Do all the bombs destroy or deathkneel doesnt trigger because it doesnt have a target to deal damage to?

No — killing one Bomb with Salvage doesn't chain-detonate the rest: with no unit of yours at that battlefield, the Bomb's death trigger has nothing legal to choose, so it's pulled off the chain before it ever finalizes and the "kill your other Bombs there" half never happens.

"Deal 2 to an enemy there" picks a unit, so it targets (Rule 355.10). Since there's no enemy unit there, the Bomb's controller has no legal choice to make when the trigger is finalized, and a trigger with no legal options is simply removed from the chain and never becomes a chain item at all (Rule 403.4) — that's not a counter, it just doesn't resolve and has no effect whatsoever. So Salvage kills exactly one Bomb and draws you 1.

Flip side worth knowing: if you did have a unit at that battlefield, the trigger would resolve normally — 2 to your unit, then all the other Bombs there die, and each of those deaths sets off its own trigger against your surviving units.