I get hives 3 times last month. Can't tell what is causing it. Not allergic to anything. Any ideas?

Non-allergic hives. Hives lasting over 6 weeks are called chronic and are almost never allergic and almost always "idiopathic" or of unknown cause. These hives are probably due to a problem controlling the cells in the skin which make and release histamine, mast cells. Bloods testing is almost always negative. Management involves giving larger than normal doses of multiple antihistamines. See an allergist.
Autoimmune hives. Allergic antibody (ige) binds to the surface of histamine-containing blood cells (mast cells) thru an ige-recptor. Many persons with recurrent hives make an autoimmune IgG antibody that binds to the ige-receptor and can trick the mast cell into releasing histamine when no allergic reaction is occuring. Lab testing is called a "cu-index" (lab corp or quest). This test may explain your problem.