(X^({1,2,3}))

(Subscript[X, 4,5,6])//FullForm

Superscript[X,1,2,3]

Overscript[X,1,2,3]

(Underoverscript[X, ({4,5,6}), ({1,2,3})])

MakeExpression[SubsuperscriptBox["x",n_],StandardForm]:=

X[({{1,2,3}, {4,5,6}})]

(Subscript[X, 1,2,3,4,5,6])

\(a,b,c\)

({{a, b}})

(X^({{a, b, c}}))

Cell[BoxData[
 UnderoverscriptBox["X", GridBox[{
    {"4", "5", "6"}
   }], GridBox[{
    {"1", "2", "3"}
   }]]], "Input",
 CellChangeTimes->{{3.970230565311392*^9, 3.970230579682558*^9}},
 CellLabel->"In[11]:="]

\(a b c\)//FullForm

HoldComplete[({{a, b, c}})]

<< Notation`

Notation[(Subscript[BX, ({{lc_, li_, lj_}})]^({{uc_, ui_, uj_}})) ⟺ (NotationTemplateTag[BX[uc_,ui_,uj_,lc_,li_,lj_]])]

ToBoxes[(Underoverscript[X, ({{4, 5, 6}}), ({{1, 2, 3}})])]