Historically people achieved this by programmatically varying the tempo. Here’s a classic MOD called black queen that showcases this technique. This is not particularly triplet oriented, rather it showcases tracking techniques that highlight the idea that a tempo is fixed is false:
Renoise itself has a delay column. This gives you 256 points of granularity between notes. You can use this column to punch in triplets.
http://tutorials.renoise.com/wiki/Effect_Commands#Delay_Column
Finally, here’s a Lua Tool that let you calculate triplets:
https://forum.renoise.com/t/new-tool-2-7-2-8-fractional-notes/34603
(I downloaded v1.03 and it works in Renoise 3.1, the API auto-upgrade was successful)
Hope this helps.