Dont hold me responsible if you blow up the engine but............

The way it normally works is you have it plumbed between the turbo and the wastegate.
Then with it closed, all the boost pressure in the line goes to the wastegate and actuates it open lowering boost.

When you open the bleed valve boost pressure is bled away (down through the 3rd port on yours) to atmosphere and the actuator therefore only sees part of the produced boost, which keeps the wasetgate closed until the turbo has produced the increase in boost for the bled air to equal the wastegate pressure................

so in summary,

closed - all pressure to wastegate= low boost
Open - boost bled away= higher boost

My advice is that if you plan to do this yourself only do it a small amount at a time and recheck (like half a turn until you get close then 1/4 turn or less as required). One way to do it is to hold the car in 4th gear then left foot the brakes to prevent your speed increasing and floor the throttle to check boost. doing it this way though you'll need to allow time for the brakes as well as the engine to cool between runs and only hold it for 2-3 seconds at a time.

Failing that get a mate in the passenger side to read the gauge

Goodluck.