% Code for the chain digraph:
\documentclass[border=1pt]{standalone}
 
% The graphics library we use: TikZ
\usepackage{tikz}

\usetikzlibrary{arrows.meta}

% Set the styles of the vertices and the edges in the digraph:
\tikzset{vertex/.style={draw, very thick, circle, minimum size=1cm},
edge/.style={very thick, -Triangle}}

\begin{document}

% Finally, here is our digraph:
\begin{tikzpicture}
\node[vertex] (v1) at (0,0) {1};
\node[vertex] (v2) at (3,0) {2};
\node[vertex] (v3) at (6,0) {3};

\draw[edge] (v1) -- (v2);
\draw[edge] (v2) -- (v3);
\end{tikzpicture}

\end{document}

% Code for the cycle-shaped digraph:
\documentclass[border=1pt]{standalone}
 
% The graphics library we use: TikZ
\usepackage{tikz}

\usetikzlibrary{arrows.meta}

% Set the styles of the vertices and the edges in the digraph:
\tikzset{vertex/.style={draw, very thick, circle, minimum size=1cm},
edge/.style={very thick, -Triangle}}

\begin{document}

% Finally, here is our digraph:
\begin{tikzpicture}
\node[vertex] (v1) at (0,0) {1};
\node[vertex] (v2) at (3,0) {2};
\node[vertex] (v3) at (1.5,2.5) {3};

\draw[edge] (v1) -- (v2);
\draw[edge] (v2) -- (v3);
\draw[edge] (v3) -- (v1);
\end{tikzpicture}

\end{document}

% Code for the multi-edge digraph:
\documentclass[border=1pt]{standalone}
 
% The graphics library we use: TikZ
\usepackage{tikz}

% The 'bending' library lets us create perfectly circular self-loops:
\usetikzlibrary{arrows.meta, bending}

% Set the styles of the vertices and the edges in the digraph:
\tikzset{vertex/.style={draw, very thick, circle, minimum size=1cm},
edge/.style={very thick, -Triangle}}

\begin{document}

% Finally, here is our digraph:
\begin{tikzpicture}
\node[vertex] (v1) at (0,0) {1};
\node[vertex] (v2) at (2,2) {2};
\node[vertex] (v3) at (2,-2) {3};
\node[vertex] (v4) at (4,0) {4};

\draw[edge] (v1) -- (v2);
\draw[edge] (v1) -- (v3);
\draw[edge] (v2) -- (v4);
\draw[edge] (v3) -- (v4);
\draw[edge] (v4) -- (v1);
\draw[edge] (v2) to[bend left=15] (v3);
\draw[edge] (v3) to[bend left=15] (v2);

% Self-loop on 1
\draw[edge] (v1.153) arc[start angle=40, end angle=320, radius=3.5mm];

% Self-loop on 3
\draw[edge] (v3.243) arc[start angle=130, end angle=410, radius=3.5mm];
\end{tikzpicture}

\end{document}