With a pair of my 3/4" spacers (like on page 1), I have seen installs in the GR with 2 7/8" deep speakers (the Focal K2 Power 165KR). There is probably a little room in the front to add a single 1/4" spacer on top of that to get you to 3.125" depth.
To fit 1" of spacers behind the door panel, you MAY need to use extended door pins. I know in the new Foresters you do NOT need EDPs to fit 1" spacers, but I never checked in the GR. And that is not to say that you would even need the extra 1/4" spacer to fit the 3.03" speaker. I don't know how much space was left behind the magnet for the Focals.
Let me know if you have other questions.