Tapping the shout key once will give you shout level 1, and holding it seems to give you either 2 or 3 (whichever highest level you achieved). Is that all there is to it? Or could you say, with a level 3 shout, choose only to go for the 2 words.

It's all in the timing.

Tapping does first phase.
Holding for 1 second does second phase.
Holding for 2 seconds does the third phase.

Listen to your characters voice; when it's at the level you want, release the button.

Unrelenting Force as example:

Level 1: Fus

Level 2: Fus Ro

Level 3: Fus Ro DAH!

My character usually acts AFTER I hold the button. Now I only hear either Fus or Fus Ro DAH!

I'll give it a try.... it didn't seem to work for me though when I tried before, at least with unrelenting force.