Looking for central Oslo hotel with great breakfast

Hotel Bristol is where we stayed some years ago and should meet your criteria.
Check out their reviews online.

I should have added we need a triple room. We saw The Bristol, but they only have doubles.

Try the Thorn hotels. We stayed at hotel Panorama. Great breakfast.

Thon hotels come in different price ranges. I have beeh happy with the budget Thon properties and was never disapoined in their breakfast offering. Thon search includes the number of people in a room: (Note search differnetiates between number of adults and number of children and older children vs young).
https://www.thonhotels.com/our-hotels/norway/oslo/

I shoulld add that when a Norwegian hotel says "single" room, the single room comes with a single person bed and smaller room than a double. Doble beds may be two single beds together.

I liked the Astoria, a budget Thon hotel a short walk from the Oslo S and KarlJohangate.
https://www.thonhotels.com/our-hotels/norway/oslo/thon-hotel-astoria/

Triple/Family

Spacious double room with a double bed as well as extra sleeping
arrangements in the form of additional bed(s) or a sleeper sofa, for
up to four people.

Triple rooms can be a bit tricky to find, they are usually a bit rarer.

Otherwise, Scandic is also a good hotel chain in Scandinavia.

Like Tammy we stayed at the Thon Oslo Panorama. There were 3 of us. There was a tiny little room with a full size bed in it (and not much more) and a pull-out sofa in the main room on which my daughter slept. It also had a washer/dryer combo which was handy after almost 2 weeks of travel. Great breakfast, good location.

November 2019 we stayed at the Thon Hotel Rosenkrantz and the location was perfect. When we booked our rooms their website said that they had received an award for Best Breakfast and I believe it. The breakfast was superb, so much variety and everything was fresh and delicious. If I return to Oslo I’d stay there again or splurge and stay at the Bristol.

We booked the Thon Hotel Cecil for our trip. It had great reviews, and was in a good location.

The Thon Hotel Cecil is my tour hotel for the RS Scandinavia tour in late May!

We stayed at the Thon Opera. Good location, nice rooms, excellent breakfast. This hotel is literally next door to the train station.

I stayed at Thon Hotel Munch, not far from the Bristol. The Scandinavian breakfast buffet is one of the chain’s claims to fame, and it’s very good. As others have said, different Thon Hotels have different prices based on amenities. Munch was toward the bottom of the range. They just remodeled and do have triples.

Scandic Oslo City was central and had a great breakfast, as did other Scandic hotels we stayed at. I hear that Thon hotels have great breakfasts too, but in Oslo, the Scandic was far easier to find walking from the train station than the Thon and so very convenient to all public transportation, trams, busses, ferries.