You should use the centre lane.

The traffic rules regarding positioning state that: "Insofar as circumstances permit, vehicles shall drive on the right-hand side of the road. On carriageways with two or more lanes in the same direction, the right-hand lane shall be used when traffic rules do not require or permit use of the left-hand lane(s)."

The middle lane is considered the right lane for this scenario when going towards Kristiansand. You should use this lane because there is no reason to be in the left lane.