As Blue said, TTL is just a way of turning a laser on and off for a laser show, without having do disconnect power from the PSU. TTL comes standard in most lab lasers. just input 0-5v into the TTL wires, thats how you use it.
If I remember correctly with my scanner it only has 2 wires, so I would guess the third wire isnt needed in most cases.
One thing I noticed with TTL, when the specific laser isnt needed in that effect and is supposed to be off, the power of it is turned right down to around 1mW. I noticed on my scanner that when the green laser is in use (which isnt connected up yet anyway) my red laser that is off during the effect gives out a very faint beam, you cant see a dot but in a somky room it can be seen very faintly.