All three responses are common. #1 is best grammatically though, using a future tense to describe a future event.

1. What do you think about today’s weather?
2. I am having dinner there, maybe sometimes, just later on?
3. I have dinner then.

Both the first and the second are corrects. What are some examples of verbs that sound very natural and spousal. If you were more formal, you might say something like “My Apologies, I have a prior engagement for dinner” and the rest would pass by your chin. We speak with each other the same way as the first speaker implies “Let them understand that this is a casual interaction”. s that true?

I have dinner can only refer to possession of dinner rather than eating it, although you can use other tenses (e.g. I will have dinner (for other people) to refer to eat.