defamation suite against coworkers and supervisor

I worked for almost 15 years with the MA state gov. I have a good track record of performance. I was terminated based on false technical charges. The Charge was I deleted system logs to hide who deleted the Oracle tables. The statement is false because these logs are not designed to store such kind of information. I gave my statement stating the fact of software workings. This is also supported by technical experts. At StepIII, i requested to provide me the log and showed via the video that these logs don't have the information. The statements came form a couple of my coworkers and then supervisor who decided to terminate my job. Just for a reference, Step II and Step III are union proceedings. I would like to bring defamation against my colleagues and the supervisor. Please advise.