1. Machen wir das- answering "lets do it"
2. Kann ich auch das machen- answering that you also can do something
3. Habe ich das gemacht - answering that you have done something

I've seen and heard this many times. Is it considered slang?
Or there is a rule to when I can put the verb in 1st position?

Of course i know that in a question and imperativ it works that way, but i thought that beside these 2 options, verb in first position can never happen in Deutsch

Thanks