1. Who's calling, please?

2. Who's speaking, please?
3. Who's this, please?
..................................
In a telephone conversation, are all correct? The same?

All are correct, but 2 isn't used much.

Yes, all three questions are commonly used in a telephone conversation to ask for the caller's identity. While they have slightly different phrasing, they serve the same purpose of politely asking the person on the other end of the line to identify themselves. The choice of which question to use can depend on personal preference or the specific context of the conversation.