I'd actually take Bergen, as it's the 'exotic' destination for a short trip. It's absolutely beautiful, although prohibitively expensive and gastro options are limited. If you're going in the Spring/Summer, consider it.

Copenhagen is Scandinavia's most international-vibe city, due to the Øresund region. If you want city vacation, take Copenhagen.

Oslo is, well ... not that interesting. Extremely expensive, small and nothing of particular interest that you can't see or do elsewhere. There's no real city vibe, just one main street for leisure and shopping and a teeny-tiny "gentrified/cool" area that isn't cool in any way. I've been to Oslo lots of times for work and family and I've never left thinking 'wow, what a great week or weekend'.