The 1 point of damage is assigned as any other trap damage (i.e., it can be prevented by thieves or the paladin). The 2 additional damage is a special effect of the trap. Because of this, it can’t be prevented by anti-trap abilities (neither by thieves nor by the paladin) even if the first damage was prevented.

(The text on the card explains that thieves cannot prevent the damage, but this applies to paladins as well. The card will be corrected in later editions.)

Note that the additional damage is assigned at the end of the round, even after the Conquering Step. That means that even if this damage eliminates the last adventurer, the party can conquer a tile before it happens. Also note that if the adventurer you hit is eliminated by monsters or by fatigue, these two damage are not dealt. (They do not transfer to the next adventurer.)

The damage is dealt even if there was no Conquering this round.

(The “At the end of the round (after Conquering)…” text on the card just specifies the timing of these events.)