BFW,
You could get a reed relay from Radio Shack to do this, too.
Its coil is 250Ω, so it'd draw 20mA from the output pin, no problem.
I think you need a rectifier diode across its coil (get that, too.)
Then attach the contacts (NO and COM) between Ground and your wire 3, either way, that part of the relay is just a switch.
Advantage: This keeps the Arduino and the camera volts/circuits separated.
[Sorry for the interruption.]
I think that the reviews-complainers' applications were likely inappropriate, these work like a charm in situations they're suited to - like yours.