1. We sang karaoke together.

Question

2. We sang together in Kareoke.
3. We sang together in a kareoke.
[Which one is grammatical?]

Answer 1

Yes, one is best. Two is not bad. Three doesn't seem right in America. We don't refer to karaoke as an entity unto itself. We might say we sang together in a karaoke BAR or NIGHTCLUB. "A karaoke" doesn't make sense to us.