\documentclass{article}
\usepackage{l3fp-extras}
\ExplSyntaxOn
\fp_set_function:nnn {foo} { x,y,z } { sin(x)+cos(y)*exp(z) }
\ExplSyntaxOff
\usepackage{xfp}
\usepackage{pgf}
\makeatletter
\pgfmathdeclarefunction*{foo}{3}{%
\begingroup
\pgfmathparse{sin(deg(#1)) + cos(deg(#2)) * exp(#3)}%
\pgf@x=\pgfmathresult pt
\pgfmathreturn\pgf@x
\endgroup
}
\makeatother
\begin{document}
\fpeval{foo(0.1,0.2,0.3)}
\pgfmathparse{foo(0.4,0.2,0.3)}\pgfmathresult
\end{document}