Fairness of assigning different reviewers for final decision on paper
When you submit a paper to a journal, you agree, in essence, to go by their rules and procedures for that paper until and unless it is rejected. It is too late, now, to object. 

The fact that the editor wanted additional views of the paper is a good thing, for the journal and its readers. Their goal is to publish high quality papers. 

And, if you are rejected, by whatever process, you have the opportunity to submit the paper elsewhere, of course.