if you look at the attached picture: the triplet is an ossia inside the music. I can scale the noteheads and the number, but I didn’t find a way to scale at least the second beam. Did I miss something?
The beam will be scaled by the size of the largest notehead under the beam, so you’re not missing anything.
It may make sense to have your main note last the full crotchet/quarter, and have the ossia as a separate (downstem?) voice. Possibly clearer for the player that way, and easier to achieve in Dorico.