Combining coverpoints

covergroup cg @(posedge clk);
   a: coverpoint v_a
   {..}
   b: coverpoint v_b
   {..}
   c:coverpoint v_c
   {..}
   combinedCovepoint: [How]??
endgroup

I have 3 coverpoints a,b & c. They all have 1 bin only. Now I want to create another coverpoint which reports 100% if any of them{a,b,c} get a hit. I believe this cannot be done via cross. Is there a way to do this?