I think you will be perfectly fine with a season 3 sleeping bag. Along the coast expect temperatures in the evening to be as high as 20 degrees Celsius and lows of around 8-10 degrees Celsius. Rallarvegen is roughly 1350m at the highest point. I would expect temperatures on cold nights to be 2-5 degrees and on good days 8-12 degrees Celsius.

Your sleeping bag will probably have a comfort of zero or below? It will handle all conditions in the south of Norway. My sleeping bag has a comfort of 5 degrees Celsius. I usually just add some wool clothing to keep warm on cold nights.

Note on rare occasions it can go below zero even in July on the top of mountain passes. But in that case, you just ensure you don’t camp at the top!