Javad,
what a Mathematica expression returns is always another - single! - Mathematica expression. Just three numbers 1 2 3
represent three(!) expressions. If you want to have the {}
removed just in the context of visibility or presentation then the above solution of Mariusz is perfect. You have to be aware that it is still a single expression:
Row[{1, 2, 3}, ","] // FullForm
(* Out: Row[List[1,2,3],","] *)
If your question is meant in a more syntactical context you probably need Sequence
:
s = Sequence @@ {1, 2, 3};
{a, s, b}
(* Out: {a,1,2,3,b} *)
Regards -- Henrik