http://themortgagereports.com/8910/harp-3-harp-2-for-alt-a-sub-prime-jum...
HARP requires that your mortgage be backed by either Fannie Mae or Freddie Mac. This clause specifically affects the millions of underwater U.S. homeowners whose mortgages are privately-held.
HARP 3.0 would likely help homeowners whose mortgages are specifically not backed by Fannie Mae or Freddie Mac.