Easy way to make triplets is to change the LPB or Lines Per Beat to 12 and the pattern length to 48. That gives you 4 beats of 12 lines, 12 beats of 4 lines (triplets) and 16 beats of 3 lines.
Another way is to use the phrases on an instrument to make it play at 3 LPB, even if your song uses 4 LPB. There are a couple of triplet presets in the phrases section.
The classic way would be to set the delay on the second and third triplet note in a 4 LPB pattern to 55 and AA. Suva says it well in this post:http://forum.renoise.com/index.php/topic/8796-triplets-in-renoise/#entry138180
You basically do this:
A-500…–
C-500…55
E-500…AA
…
A-500…–
C-500…55
E-500…AA
…
A-500…–
C-500…55
E-500…AA
…
There’s a handy tool for quickly laying down a bunch of fractional notes, triplets, fifths or whatever:https://forum.renoise.com/t/new-tool-2-7-2-8-fractional-notes/34603 Works in 3.0.1. Right click in the pattern editor/track/fractional notes.