Can I add a battery to some camera device?
Probably not without risk.

It MAY just work well enough, but more likely it wont.

More information needed, but, the battery is probably a LiIon (Lithium Ion) and the 3 wires are probably either
+ve, -ve, temperature sensor OR
+ve, -ve, dark and nasty special circuit.

If the 3rd wires is a temperature sensor it will usually be s NTC thermistor. Placing two in parallel makes it appear that the battery is too hot.
Disconnecting the feed from one leaves it unprotected.  

If the 3rd wire is connected to a special circuit, connecting two in parallel may make nothing happen or just about anything happen. Not usually recommended.

In either case, parallel connecting  two LiIon batteries can cause, in some cases, enough  flame and smoke inside your camera to make it look like a small firework device. This is usually not what you want to happen. .