Riffle an element with +1 every time

Create a list to riffle with,

Riffle[list, Table[StringJoin["X", ToString[i]], {i, 1, Length[list]}], 3]

{{1, 2, 3}, {4, 5, 6}, "X1", {7, 8, 9}, {10, 11, 12}, "X2", {13, 14, 
  15}}

Module[{i = 1}, Riffle[list, "X", 3] /. "X" :> "X" <> ToString[i++]]

{{1, 2, 3}, {4, 5, 6}, "X1", {7, 8, 9}, {10, 11, 12}, "X2", {13, 14, 15}}

Riffle[list, Table["X" <> ToString[i], {i, Ceiling[Length[list]/3]}],
  3]

{{1, 2, 3}, {4, 5, 6}, "X1", {7, 8, 9}, {10, 11, 12}, "X2", {13, 14, 15}}