You're one of those who just read the headline, right?
He hasn't been sentenced to 21 years. He has been sentenced to 21 years + preventive detention. That means that after the 21 years there will be a court hearing that decides whether he is still considered a danger to the public. If so, he will be kept in prison for another 5 years. After those, there will be another hearing and so on. It's highly unlikely that he will ever see the light of day again.
About this "comfortable 3 room cell". That was during the trial as he couldn't be let together with other inmates, therefore all kinds of stuff (training facilities, bathroom) had to be in his cell to prevent him from brainwashing other inmates. Norway's approach to that whole terror act ("We react to terror with more democracy") is just so adorable.
Well, just watch how we manage with life imprisonment