You can install our site as a web app on your iOS device by utilizing the Add to Home Screen feature in Safari. Please see this thread for more details on this.
Note: This feature may not be available in some browsers.
I had a ton of trouble finding a booster seat that worked well in the model 3. Lots of them fit, but it was easy for them to slide over the recessed buckle in the seat. Then my daughter (6) wasn't able to buckle herself in, and I'd have to shift the seat. (Which probably wouldn't do great...
I have the Chicco Fold&Go iSize seats in my M3 for my 6yo twins. Isofix mounts for seats, then regular seatbelt.
The family car is a BMW 5 touring, which has their "main" seats (Joie) - the Chicco's generally live in the frunk & underboot storage (where they fit nicely!) and come out when I need to ferry them at short notice. 1 min job to unfold & fit.
Took the twins to Wales in the M3 during the summer - they were happy in the Chicco's for the 5 hour journey. The seats have a couple of recline levels, but not to the degree of the Joie seats.