Will Demacian Illuminators buff themself once they and two others unit does and I use Sett to save them?

Yes — the trigger fires once and Illuminators can buff itself, as long as it's still on the board when that trigger resolves.

Two things to sequence. "When one or more friendly units die" fires one trigger no matter how many units die, so 2 deaths and 3 deaths are the same: a single Buff to hand out. And Sett's legend (The Boss) is a replacement effect — the saved unit is healed, exhausted and recalled instead of dying, so it never counts as a death at all (FAQ #11835). If you save one of the three, the other two still die and the trigger fires normally.

If the unit you save is the Illuminators itself, it's recalled to Base — still in play, so it's a legal choice for its own "buff a friendly unit," and since The Boss spent its buff to pay for the save it no longer has one, so the buff actually lands (Rule 702.3.a). If it isn't saved and genuinely dies, the trigger still resolves, but Illuminators is in the trash and can only buff a different friendly unit.