As others have said: wifi can be shared/swapped about. You may also be surprised how long you get data roaming for: because of the length of the fjords we were often able to use our own roaming service.
We usually stay in the Ibis budget, and they do rooms for 3 from £45: but it depends on when you book. There is also quite a cheap travel lodge in Newbury. Or the airport premier inn isnt badly priced - again prices fluctuate a lot!
Parking is worth booking sooner rather than later as it doesn't get cheaper the closet you get. There are loads of options. We have used CPS, Parkside Meet and Greet (through Holiday Extras) and hotel and parking packages (also through holiday extras).
I would recommend planning activities in each port and booking them so that you aren't just wandering around not sure what to do with bored teenagers complaining 😂 Whats In Port is a great resource. The Olden Skylift is popular with all ages, Stavanger has a Petrol Museum that the 3 year old would probably love, and Rib tours etc that the older ones might enjoy. Alesund had a great aquarium. Lots of these things can be done cheaper yourself by planning and pre-booking. P&O do offer some good excursions though, and they are a bit more than doing them yourself, but it all depends on the type of activity. You can also do things like take your own paddle boards or do free hiking etc. The more you plan, the more fun you will have.
In regards to the teens, I would recommend You Tube: there are loads of videos and reviews of the ship/resorts that might help make it more exciting. Also you could get them involved in the planning.