Omitting article before "evaluation"
The first sentence
This talk focuses on an evaluation of....
means the talk is about a particular evaluation that has been made.
The second sentence
This talk focuses on evaluation of....
means the talk is about the process of evaluating....
The third sentence
The focus of this talk is an evaluation of....
means that the talk itself is making an evaluation of something.
and so I would say both the second and third sentences change the original meaning.