@JasperHabicht Is the tuple of a fixed length? Because then you can do
\documentclass{article}
\begin{document}
\ExplSyntaxOn
\fp_new_function:n { extractonefromthree }
\fp_set_function:nnn { extractonefromthree } { a,b,c } { a }
\fp_eval:n { extractonefromthree(1,2,3) }
\ExplSyntaxOff
\end{document}