[Jethrow]

My cannistor is from a VS Commodore and it has 3 outlets on top.

FUEL - goes to fuel tank
VAC - goes to intake manifold for a vacuum signal/source
PURGE - goes to throttle body inlet tube upstream of TB, but after filter.

This allows the tank to be vented to an outside clean air source when the engine is idling or has vacuum in the manifold. When engine is off, no venting, hence no fumes escaping. Also this is not an air leak into the engine as the fuel tanks are sealed.

I think for some newer system there is only two lines as the vac line is also the purge line but it is controlled by the ECU with a solenoid to only open, to allow venting, when the ECU desires. This to chase that last bit of emission control, especially at startup.

HTH