In the March 6, 2018 update for Storyline 3, we restored compatibility with Storyline 360 project files that have course completion triggers. This means you can now open Storyline 360 projects that have course completion triggers, and those triggers will work in Storyline 3 (details here). However, Storyline 3 doesn’t have a completion trigger feature, so you can’t edit them or create new ones.
To be specific, you can’t edit the status for existing completion triggers in Storyline 3. The status is grayed-out as shown below. (You can edit when a trigger occurs, such as when learners click a button.)
To edit completion triggers or create new ones, open the project file in Storyline 360, which is the latest version of Storyline and part of Articulate 360.