Just so you know, a 3" on a normally aspirated motor will be extremely loud, even with a resonator, since the turbo provides significant muffling. For most 4-cylinders, anything over about 2.25" will be raspy, loud and possibly unbearable under full throttle. Plus you'll lose the scavenging benefit from having a properly sized NA exhaust with the right exhaust velocity (A NA car with a too-large exhaust can actually lose power. I doesn't really matter with a turbo, since the exhaust before the turbo is restricted anyway, you just want low pressure after the turbine). It will not give the pleasant growl that it will with a turbo in the way. You might be able to get away with a 2.5"
The Galant GSX should have the same AWD floor pan as the VR-4, unlike the FWD versions, so you should be fine in that respect. The downpipe location off the stock cast header might be slightly different, although a test fit would be the only way to tell - If it's slightly higher up, you might be able to have a welded "adapter" made. If you have the exhaust "lying around" do a test fit.
If you don't mind losing your eardrums, and still want to use the 3" exhaust, another "hack-job" option would be to acquire a 14B or 16G exhaust housing, weld up the hole where the CHRA goes and remove the wastegate and flapper, and bolt it all up using a turbo manifold and O2 sensor housing. It will probably be fairly restrictive though.
