I do a lot of songs that have triplets in them. It would be nice if theory tab allowed users to enter triplets.
You can emulate triplets by setting the time signature to 3/4 or 6/4 and upping the tempo quite a bit. One beat becomes a triplet 8th, quarter, or something similar, and one measure is either 1 or 2 beats, based on the time signature.
However, this is best suited for songs that are in 6/8 or 12/8. Hookpad 1 (the one currently used for submitting to Theorytab) does this.
Now, with Hookpad 2, you could just temporarily change the time signature of a single measure so that it's in 12/8 or something similar, assuming you want 8th note triplets intermixed with quarter notes. If you want 8th note triplets with regular 8th notes, you could do 24/8, etc. without having to change the song's tempo.
EDIT: it seems you can only go up to the equivalent of 12/8 counting in 12. Maybe you could circumvent this by separating the tripleted measure into at least 2 separate measure, and having the triplets be in its own measure. Messy, but it should work.