Not exactly. Triplets have nothing to do with time signature. Normal timing is 4/4; that's four quarter notes, or eight 8th notes, per bar. In your example, 12/8 means you're lenghtening the bar with four extra 8th notes. It's the same thing as 6/4 timing (six quarters per bar). Basically it would be like playing one and a half bars.
Triplets are a whole different thing. Normally where you'd fit only two 8ths per quarter, when playing triplets you're adding in a third note – but the time between quarters doesn't change. So all three 8ths would be slightly less than a regular 8th by 1/3.
Say you're playing a song which has three seconds between quarters. That means each 8th note would be 1.5 seconds long. With triplets, though, you'd be adding another 8th note between quarters but they all three have to fit into those three seconds, so each 8th would be only 1 second long.
This also applies to triplet 16ths (three 16ths per 8th note instead of two) and triplet quarters (three quarters per half note instead of two).
Check it out – This is a metronome which plays 8th notes (select any tempo) :
http://www.metronomebot.com/subdividing-metronome-duple.htmlAnd this is a metronome which plays triplet 8ths. Set the same tempo as you used on the one above, and notice the difference:
http://metronomebot.com/subdividing-metronome-triple.htmlI hope this helps.
