Front Speaker Fitting Advice

Yes they will fit but you can't just use generic MDF rings, they need to be specifically for the A3. The A3 has a strange 3 point offset arrangement where the speakers bolt to the flimsy door card, so the adapters have to sit over the mounting posts to seal correctly around the 4" aperture, and the screw holes have to be accesible in order to attach the ring. Then a second bigger ring screws to the first to provide the larger aperture for the 5.25" speaker. This also has to have some depth to prevent the speaker cone fouling the first ring as the Rainbows cone, like many aftermarket components, sits proud of the speaker cage. You can make your own, PM me if you want the templates.
If you don't want the hassle of rewiring the doors then the crossovers can be hotglued to the doorcard, you just need to find a spot where they don't foul the window mechanism.