3 port boost solonoid??
 

What is the purpose of these(what do they do?) and are they worth having?

They are for better boost control. Usually used in conjunction with a re-map as going from OEM 2 port to 3 port without a re-map will mean the map is not suited for it and all to pot.

HTH.

If running above 1.2/1.3 bar they are better an controlling the boost.
They are standard fit on pre96 cars.

i have a spare on from my 95 wrx if you need one

I couldn't be able to up the boost above 1.11 bar with the stock 2 port boost solenoid. Now i'm able to set the boost level whatever I want with a 3 port solenoid :D Its a MY98