The SPT is the exact same between the STI and WRX (08+ hatch). Definitely not 3". None of them are (including the single exit hatch SPT). This is my second SPT, one came off a WRX and one came off an STI. Same thing. The exhaust actually flares out at the flanges and gets wider. The only 3" section is the few inches after the midpipe and before the muffler.
Here's the stock image. You can see it doesn't "neck down" at the flanges.