I think the 05 Sti and WRX have an in-line sensor, white or grey in color on that hose. that is supposed to detect a leak and throw a code.
The Sti has dual hoses venting the breather cover for the PCV. There is a PCV cover under the overflow tank. One of the two hoses would be blocked if you did a good job blocking hose #3 to the bottom right.
I am new to Sti PCV routing, so I never traced the second set of PCV hoses. Trace it and block it.
Most likely even when you're done with blocking the hoses, you're not done yet. You will have to turn the crank manually slowly, as you feed air into the test rig, to find the valves closed position.
Depending on which turbo you have, it may let air go directly into the exhaust, from the compressor housing. It may not do that hot, but do that cold.