First of all, when setting up the game, make sure you have three powerups selected. They should show right below the score . Be careful not to hit PLAY until you have checks on 3 of them. It doesn't matter what 3 you select.
Once the game begins, the powerups are randomly generated and look different from the standard shape outline before you place them. They're outlines with a glittery/sparkly center. (Sorry for the horrible description.)
When you get one of these, choose the a vertical position and line it up to one side. It's important to pick the vertical shape so you save room for all 3 powerups on one line. Now, don't complete that line - build upwards until you get another powerup. Once you get the second one, select the vertical shape if possible, but more importantly, put it on the same line as the first. Again, build upwards until you get the third power up outline. Place this on the line with the others and now finish that line. It will trigger all three specials at the same time.