If opponent is attacking with sleuth scouts and ds wins edge can the droid be put into play appart of the engagement even though he has a cost of 3? now he is not being declared as defender he is just jumping in the middle of the fight

Yes because you are past the declare defenders step.

Yes because you are past the declare defenders step.

It's not even because you are pat the declare defenders step, but just because it wasn't declared as a defender. Relevant in that Backstabber (or any other future units with a similar ability) could be played in the Action window between declare attackers and declare defenders if you wanted to.