Cfxtrjtrk

I wish to use the overhead brace, bracing over the three items 8, 3, 11 from the bottom of 2.

Here is a template:

    documentclass[11pt,urlcolor=blue, linkcolor=blue]{article} 



    usepackage{sseq}

    usepackage[all,cmtip]{xy}

    usepackage{tikz-cd}

    usepackage{tikz}

    usetikzlibrary{matrix}

    usetikzlibrary{decorations.markings}





    begin{document}





 begin{figure}[!h]

begin{center}

begin{tikzpicture}[scale=0.8]

node at (0,5) {1};

node at (0,4.5) {2};

node at (0,4) {3};

draw[->] (0,3.5) -- (0,2.5);

node[right] at (0,3) {};

node at (0,2) {4};

node at (0,1.5) {};

node at (0,1) {5};

node at (0,0.5) {6};



draw[->] (-0.5,3.5) -- (-3.5,2.5);

node at (-2,3) {};

node at (-3.5,2) {7};



node at (-4,4) {8};

draw[->] (-4,3.5) -- (-4,2.5);

node[left] at (-4,3) {};

node at (-4.5,2) {9};

node at (-4,1.5) {};

node at (-4,1) {10};





node at (4,4) {11};

draw[->] (4,3.5) -- (4,2.5);

node[right] at (4,3) {}; 

node at (4,2) {12};

node at (4,1.5) {};

node at (4,1) {13};







end{tikzpicture}

end{center}

end{figure}



    end{document}

I have not simplified your code which can be simplified in several different ways. I used the positioning library (page 229 of the manual 3.0.1a) to position nodes 2 and 1 relative to node 3.
I named a few nodes by their value.
To use the braces, I used the decorations.pathreplacing library. (page 581 of manual 3.0.1a)

  documentclass[11pt,urlcolor=blue, linkcolor=blue]{article} 

%    usepackage{sseq}

%    usepackage[all,cmtip]{xy}

%    usepackage{tikz-cd}

    usepackage{tikz}

%    usetikzlibrary{matrix}

%    usetikzlibrary{decorations.markings}

usetikzlibrary{positioning,decorations.pathreplacing}



    begin{document}

 begin{figure}[!h]

begin{center}

begin{tikzpicture}[scale=0.8]



node at (0,4)(3) {3};



node[above =of 3] {1};

node[above=5mm of 3]{2};

node at (-4,4)(8) {8};

node at (4,4)(11) {11};

draw[decorate, decoration={brace,raise=2pt,amplitude=10pt}](8.north west)--(11.north east);

draw[->] (0,3.5) -- (0,2.5);

node[right] at (0,3) {};

node at (0,2) {4};

node at (0,1.5) {};

node at (0,1) {5};

node at (0,0.5) {6};



draw[->] (-0.5,3.5) -- (-3.5,2.5);

node at (-2,3) {};

node at (-3.5,2) {7};

draw[->] (-4,3.5) -- (-4,2.5);

node[left] at (-4,3) {};

node at (-4.5,2) {9};

node at (-4,1.5) {};

node at (-4,1) {10};

draw[->] (4,3.5) -- (4,2.5);

node[right] at (4,3) {}; 

node at (4,2) {12};

node at (4,1.5) {};

node at (4,1) {13};

end{tikzpicture}

end{center}

end{figure}



    end{document}