AI, which isn't always 100% accurate, offers these suggestions...
"To activate and create the triplet sign in NCH Crescendo, follow these steps:
Select the Tuplet tool from the Note Toolbox.
Click the first note and then the last note of the group you want to make into a triplet. This should create a triplet bracket with the appropriate "3" marking above or below the group by default.
If you need to customize or edit the triplet, right-click on the number, bracket, or a note inside the tuplet to bring up the popup menu. Here you can adjust the tuplet value, appearance, and time ratio as needed.
If the triplet sign is not activating:
Make sure you have the correct number of notes selected and that you are using the Tuplet tool, not just entering three notes and expecting them to be grouped automatically.
Try restarting Crescendo or your computer in case of a temporary glitch, as there have been reports of the feature suddenly stopping during editing.
As a workaround, you can try selecting the notes you want to become a triplet and then activate the Tuplet tool again to force the grouping.
If the problem persists, check if your software is up to date or consult the official [help manual].
Note: Triplets (and other tuplets) are managed through the Tuplet tool, not by simply adding notes and editing text as in some older methods.
If you have already created several triplet groups, you should still be able to make new ones unless there's a software or file-specific glitch. If it’s a persistent bug, contacting NCH tech support or checking user forums for similar reports can help."