Written up and need to reply within three days

I work in the hospital setting as an shift supervisor.  My boss left the system in October of last year and her boss,Cindy, my internum boss, set me down with HR to discuss next steps for me and caution me on a close friendship with someone else's' direct report, Melissa, who is a technical specialist in the same department.  Due to an outside consulting firm potentially rearranging the organization within the department my old bosses job has never been filled.  Multiple people including me, Melissa, and her boss Becky have stepped forward to fill the work of the vacant position.  Since Becky was only knowledgeable about one area of the department, and Melissa and I work in the areas of the department left with no management, we took the majority of the workload with Becky advising.  This has caused the perception that Melissa and I run the area which is the cause of my write up. Melissa is not in management but only a technical supervisor.  I am seen as working too closely with her and sharing information and allowing her to make the same level of decision that Becky and I do.  "The job of a supervisor requires a clear distinction of positional authority that is maintained at all times while on the job.  The supervisor position also requires preservation of all sensitive information/data/knowledge is held in strictest confidence and not shared with anyone outside the leadership team."  From my point of view we are just trying to get the work done.  This has been hard for me to maintain with the absent of a management team member and the validation fo a new LIS system which Melissa is highly part of as a system team lead for one area I cover.  With this write up I cannot apply for another job within the hospital network for at least six months. I have three business days to response to this written with an action plan and any comments to what was written.  Not sure how to respond and need advice.  Only coming up with asking for a further explanation sit down with Cindy,me and a HR rep.  I want to give them my action plan and ask clarifying questions, but most everything I am thinking about comes up a little defensive.  To the point, I just want to know are they willing to work with me to continue my career path or are they just blocking my career here.