[S-mk3-B]

So first off are you determining the 3psi reading from the stock gauge, or an aftermarket one? So it looks like you checked all the basic diagnostics as leaks and vacuum hoses, you didn't mention it but double check your intercooler piping too, b/c that can add to your problem. And if all that checks out then I would probably suggest inspecting your exhaust pipings, cause if it's clogged/backed up at all then that could create back pressure. And after that it just might be a bad wastegate and etc... Hope this helps!