Angelito, the immune system is very complex. Those 3 tests identify which antibody groups are potentially attacking platelets. There is a lot of good general information on Wikipedia:
... "Most often these antibodies are against platelet membrane glycoproteins IIb-IIIa or Ib-IX, and are of the immunoglobulin G (IgG) type." ...
en.wikipedia.org/wiki/Immune_thrombocytopenic_purpura
I suspect that your doctor was trying to eliminate familial/congenital Thrombocytopenia as a possible diagnosis. One would expect all three to be negative in that situation. See the end of this study's abstract:
"Prospective evaluation of a new platelet glycoprotein (GP)-specific assay (PakAuto) in the diagnosis of autoimmune thrombocytopenia (AITP)"
www.ncbi.nlm.nih.gov/pubmed/15726595