Let's have a coffee, My treat or It’s on me or Be my guest

Hello

Which one of the options is OK?

Let’s have a coffee,

1- My treat 2- It’s on me 3- Be my guest.

Thank you

Let’s have a coffee, my treat.

  • spoken by one person indicating that they are both suggesting they have a coffee and offering to pay for it.

Let’s have a coffee. It’s on me.

  • spoken by one person indicating that they are both suggesting they have a coffee and offering to pay for it.

Can I help myself to a coffee?
Be my guest.

  • spoken by two people. The first asking if they can take a coffee, the second indicating that they are happy for the person to help himself.