I'm not a fan of using factory 5.25" F and 4" R. Personally, I installed a component 6.5" system using provided brackets in my front doors. I gutted the factory tweeter and managed to fit the new one in the factory location with pretty much no room to spare. I then mounted the crossovers in the lower right portion of the door. I can't remember if I had to trim the metal on the door. I probably did. I also had to cut out the Styrofoam on the door card to clear the crossovers. Both the tweeter and crossovers had to be mounted with custom brackets. Unfortunately, I must have forgotten to take photos.
You can actually fit 6.5" coaxial (two way) in the rear if you trim a LOT of material. I used an air saw. It barely fit. I had to trim the plastic off the tweeter so the door card would sit flush. Is it worth putting a two way or such large speakers in? No, I don't think so. The location of the factory rear speakers are very poor. No matter what you do, it's going to sound like shit back there from the drivers perspective. This was my part time job at Best Buy, so I got the speakers for cheap and had plenty of down time to see what I could fit back there.
Rear door of a 2004 WRX. Outline to fit 6.5" Infinity Reference 2-way speakers.
Speakers installed after door cutting and water "proofing" with dynamat. Photo was taken before I realized I had to trim the tweeter.
I just re-read your post. I should probably explain more. If what you mean by 3-way is separate base, mid, and tweeter speaker, I am unsure if you can fit oval type speakers in our doors. You may, but I personally have no experience with them. AFAIK
, most people stick to component (separate tweeter) or coaxial (two way). If you go the separate tweeter route and you have factory tweeters (again, I'm pulling from fuzzy memory), I believe you have to gut part of the speaker wire. I think Subaru does it differently than aftermarket. You need the full signal which goes to a passive crossover. In this box, it filters the signals and sends a set to the tweeter and the other to the speaker (mid/base). That means you have to trace the wires far enough back to find where Subaru does its split. Hopefully, someone that's done this more recently than I have can chime in.
As for coaxial, the "crossover" is built in. No box needs to be mounted (not provided) and the wires from the HU just go to the speaker itself. Obviously, components will sound better than coaxial. But it also has a lot to do with tweeter location. Factory location of the tweeter is OK - not great.