As far as i know the 3rdport is a vent to atmosphere, when your boost solenoid wants to open the vnt it can only shut off vac to actuator, but then it needs to let air in to the actuator to let it open.
It may just be a diffent design so you can run a vent pipe higher up in engine bay to keep cleaner/dryer.
With the car idling does the actuator pull the vnt arm fully until it hits the small stop screw?
Idle the car and pull the pipe off the turbo actuator( if you can, probably tight on) you will see the actuator open out and arm move about 10mm, then refit pipe and it should pull back in 10mm all the way to the stop screw.
If it doesnt you may have leaks on vac pipes to and from vnt valve , dodgy valve, low vac from pump, stuck vnt vanes( stuck open, low boost position)
Dont forget egr stuck open will lose power down low also, as will boost hose leaks.