What are your concerns with this disposition? Do you just want to continue to serve because you like serving, or is it a financial issue, or something else?
I am not sure what you mean by getting your High-3 for MSgt. Your High 3 is based on the average of your actually received pay. So, you will get the same amount if nothing changes for you, essentially, as you would have had you retired for length of service. With your current disposition, you will receive High-3 pay based on your years of service times 2.5%, the same as a LOS retirement.
AF consideration of appeals to SAFPC after a formal hearing are SLOW. I just got notified in a case that I submitted more than a year ago that they were just now getting to considering the appeal. If you want to, it is likely you can drag out the process for a long time.
If you really just want to stay, I would look into LAS (though, depending on your chain of command support and manning in your AFSC, this may or may not be wise...that is, I would consider the likelihood of approval/disapproval vice the time an appeal would take also keeping in mind you goals; the AFI indicates, as Ruptured Duck alluded to, that 20 years is the ceiling for LAS. This is true as to what the LAS states, but I have had some conversations with the PEB President that indicates that this might not be how they are actually adjudicating these LAS cases...would look into that before making any decisions about LAS).
Good luck!