Here are the one-word completions for each dialogue:

**Dialogue 1**
A: We're going to the disco. 'How about coming with us?
B: That's a **great** idea!

**Dialogue 2**
A: We're going on a day out to Oxford. Why **don't** you come with us?
B: I'm sorry. I **can't**. I've got a test on Monday.

**Dialogue 3**
A: We're going to the cinema tomorrow. **What** about coming with us?
B: OK, **great**.