1. What are we going to talk about?

2. What will we talk about?
3. What shall we talk about?
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does #1 mean #2 or #3?
Does #1 mean that we had already a topic beforehand and we want to talk about it today? Does #1 mean that we decided to talk about a topic before?

All 3 questions mean the same thing.

Without context, it’s hard to say if the topic of discussion is already known or if the discussion will be whatever people feel like discussing at the time.

The three phrases "What are we going to talk about?", "What will we talk about?", and "What shall we talk about?" are all similar in meaning and can be used interchangeably in most situations. They all imply that there is a desire or need to discuss a topic, but without specifying the topic itself.

In the given context, it is difficult to determine the exact meaning of the phrase "What are we going to talk about?" without additional context. However, typically it implies that there has been some prior agreement or understanding that a discussion will take place, but the specific topic has not been decided yet. It suggests that the participants are looking for a topic to discuss or are seeking options for a conversation.

It's important to note that communication can vary based on the speaker's intention and the context in which it is used. If you are unsure about the meaning or intent behind a particular phrase, it is always best to seek clarification from the speaker to ensure you understand their intended message.