SequenceSplit[list, {a_, b_, c_} :> {{a, b}, c}]

    (* {{{a, b}, c}, {{a, b}, d}, {{a, b}, e}, {{a, b}, f}, {{a, b}, g}} *)

Or

    MapApply[{{#1, #2}, #3} &]@Partition[list, 3]

     (* {{{a, b}, c}, {{a, b}, d}, {{a, b}, e}, {{a, b}, f}, {{a, b}, g}} *)

Or 

    MapThread[{{#1, #2}, #3} &, Transpose@Partition[list, 3]]


    (* {{{a, b}, c}, {{a, b}, d}, {{a, b}, e}, {{a, b}, f}, {{a, b}, g}} *)