skirmisher sabre damage is DoT or per attack

DoT, but red was reworked to be on-hit/DoT.

At level 18 it deals 162 total true damage over 3 seconds. That means it deals 54 damage per second.

Every time you do damage to the target the duration of the DoT effect is refreshed to the full 3 seconds. It won't stack but you can keep reapplying it until the 4 seconds of the smite debuff is expired for a theoretical maximum damage of 378.