The triplets feature on the TD-3 does not necessarily allow you to insert triplets at certain points within your sequence, this more acts as a way of playing your pre-recorded sequence in the style of triplets.
The TD-3 has two modes - 8 note and 8 note triplet. By default, any created pattern will be played in 8 note timing as per the TIME MODE settings you have configured whilst creating the sequence. To change to 8 note triplet mode, simply hold down FUNCTION and press UP. Your pattern will then play back with triplet timing.