p.s. If you're doing live recording into Composer to get this notation, you'll probably want to try setting the Transcription options (Setup/Transcription options) to anticipate two voices.
TranscriptionTwoVoices.png
However, if you have triplets as in your example that don't get transcribed as two voices, you can delete the notation and re-enter it using the tool palette and mouse as described below.
ttfn,
Sherry