DouKaku?

function doukaku73_bad(a, b, c){
  var f = function(x, y){ return x - y }, v = [a, b, c].sort(f), i = 0, d;
  for(; v[0] < v[1] && v[1] < v[2]; i++){
    v[0] += 2, v[1]--, v[2]--;
    if(v[0] > v[1]) v.sort(f);
  }
  return i + v[1];
}
/// ↓ ///

function doukaku73(a, b, c){
  for(var v = arguments, r = [], i = 3, j; i--;)
    r[i] = (v[i] - v[j = (i + 1) % 3]) % 3 ? Infinity : Math.max(v[i], v[j]);
  return Math.min.apply(null, r);
}