Making diagrams

Welcome to TeX.SE! If you are not insisting on xymatrix, you could try

\documentclass{amsart}
\usepackage{tikz-cd}
\begin{document}
\[
\begin{tikzcd}[row sep=4em]
A \arrow[rr, "f"] \arrow[rd, "g"swap]& & B\\
& C \arrow[ur, "h"swap] &
\end{tikzcd}
\]
\end{document}

EDIT: switched to relativistic ;-) units, big thanks to @egreg.

Using the xymatrix package you can see an example of little guide for this package: https://www.jmilne.org/not/Mxymatrix.pdf. I have created with this package, for your request, a simple code.

Picture with equilateral triangle.

\documentclass[a4paper,12pt]{article}
\usepackage[all]{xy}
\begin{document}
\xymatrix@C-=0.5cm{
A 
\ar@{->}[rr]^f \ar@{->}[rd]_g & & B \ar@{<-}[ld]^h\\
&C&\\
&& 
}
\end{document}