I refer to the question after the first major edit was done, where a das was added in each example at specific positions that made the sentence structures change and, therefore, the previous answer by @tofro obsolete. The current sentences are:
- Machen wir das
- Kann ich auch das machen
- Habe ich das gemacht
You want to know why the verb is in the first position and not in the second one as you would expect in case of declarative statements.
The answer is: none of the examples is, now that the edit is done, a proper declarative statement in German. And since the punctuation marks are missing, we have to interpret what else they can be.
There are two options for 1.: It can be what @tofro in a comment referred to as an imperative to the self (Let us do this!). Some call this first-person-plural imperative an adhortative. Or it can be a question (Will/ shall we do it?). Examples 2. and 3. can only be interpreted as questions (Can I do also this one? and Did I do this?).