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: 
Working with the editor, until that process is finished, is probably the only thing to do at first. It will undoubtedly be a long process, with delays, so patience and persistence will be necessary. Try not to let the mobbing get to you, and try to remain calm and professional at all times, despite any pressure others may exert. 

If the editors, in the end, are unable to come to a resolution you accept, then the options will be (essentially) to escalate the situation or to let it go. But making an action of that kind while the editors are still working would be premature, in my opinion.
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.