When the Omegawolf attacks a target that is guarded by the Huntsman, which of the three (Omegawolf, target, Huntsman) die?
The Omegawolf usually can't be killed at night; does that apply to this situation, and what happens to the target and the Huntsman?
That is correct. The Omegawolf won't be killed in this situation as they are immune to the kill.
The Huntsman will still sacrifice themselves though, and the target will remain alive - so in this situation only the Huntsman will die.