Question: Some people erased three author names including mine from a paper previously submitted (and rejected by three different journals) and got it accepted by Case Reports In Emergency Medicine. 

I presented the publishing editor of the journal multiple clear proofs of the plagiarism. Although the journal declares that it complies with COPE, the publishing editor is dragging feet not to publish a retraction about this article despite the proofs. Instead, they are trying to overlook the situation by involving the politically driven hospital administration from where the paper was submitted from. This in turn caused violation of my rights both as a human and as a researcher. I am currently subjected to serious mobbing by the hospital administration secondary to the inconclusive behaviour of the journal. This mobbing includes the change in my workplace and an official warning because "I ruined reputation of the hospital by contacting the publishing editor".

I also sent a message to COPE but there is no response. 

Would anyone suggest any further action?

Answer: 
I always get down voted for saying this, but outside of Academia, if there is a very cut and dry case of someone trying to take ownership of works that you hold copyright in, then it would go to court and be handled in about a month or so (in Germany).

I appreciate that going to court is something that is considered the absolute last option in Academia, rather than the first option everywhere else, but in my opinion this is a terrible mistake. We don't tell academic's to just "research the problem and fix it yourself" when their car breaks down. We don't ask academics to "go through the correct channels" if a teacher is caught abusing a student. No, we extend our arms out to the rest of society and get the professionals to sort it out. People who are experts in dealing with these issues. But for whatever reason, many academics think that because they are intellectual, they must by default know everything there is about intellectual property law, and can handle these sorts of issues in their weekly committee meetings. Or perhaps they think they can handle the issue with more grace and finesse than the justice system. Maybe. But I have heard the same logical argument used by both the military and religious groups, who ultimately put justice secondary to some other objective - like saving face or maintaining the hierarchy of power.

Regardless of whether the journal/hospital does come to the right decision in the end, by not exercising your right as the copyright holder to justice through the justice system, you are handing unwarranted power to the publishers and academic establishment handling your case, and disenfranchising the people who have spent their lives thinking about copyright disputes and their fare/proper outcomes. All to avoid some perceived stigma over using real lawyers.
Search for legal help from an expert in I.P. , expose them publicly and hope for the best.
This doesn't mean that you'll be able to prove the rights if you didn't took enough precautions to protect your paper and ignorance by this side is hard to prove in court.
The next time though, you can publish your own thesis/essay/book and get an ISBN with total rights and you might still be able to publish your work before it gets published since "accepted" doesn't exactly mean "released" or "printed"(this isn't clear in the question).
After years of oppression, I find this as the only way to get proper recognition since many of those cheap "book makers with ISBN" can print your work "in hours" and you can begin to spread your own knowledge at your own pace and people will think twice before abusing with your intellectual property.