generating LaTeX tabular code during compilation

Actually the LaTeX format does have loops \@tfor etc, but in tables TeX is in a bit of a delicate state so here I have just use plain expansion.

\documentclass{article}


\def\listheadings{%
\expandafter\xlistheadings\regionlist,\relax,}

\def\xlistheadings#1,{%
\ifx\relax#1%
\expandafter\\%
\else
&\textbf{#1}%
\expandafter\xlistheadings
\fi}

\def\listbody{%
\expandafter\xlistbody\regionlist,\relax,}

\def\xlistbody#1,{%
\ifx\relax#1%
\else
\textbf{#1}%
\gdef\thisrow{#1}%
\expandafter\xlistdata\regionlist,\relax,%
\expandafter\xlistbody
\fi}


\def\xlistdata#1,{%
\ifx\relax#1%
\expandafter\\%
\else
&\csname origREGION\thisrow destREGION#1\endcsname
\expandafter\xlistdata
\fi}


\def\preamble{\expandafter\xpreamble\regionlist,\relax,}
\def\xpreamble#1,{%
\ifx\relax#1%
\else
c|%
\expandafter\xpreamble
\fi}

\def\tablestart{%
\edef\temp{\noexpand\begin{tabular}{|c|\preamble}}%
\temp}

\begin{document}

\newcommand\regionlist{ONE,TWO,THREE}

\def\origREGIONONEdestREGIONONE{1/1}
\def\origREGIONONEdestREGIONTWO{1/2}
\def\origREGIONONEdestREGIONTHREE{1/3}

\def\origREGIONTWOdestREGIONONE{2/1}
\def\origREGIONTWOdestREGIONTWO{2/2}
\def\origREGIONTWOdestREGIONTHREE{2/3}

\def\origREGIONTHREEdestREGIONONE{3/1}
\def\origREGIONTHREEdestREGIONTWO{3/2}
\def\origREGIONTHREEdestREGIONTHREE{3/3}


\tablestart
\hline
\listheadings
\hline
\listbody
\hline
\end{tabular}

\end{document}

Here's a solution with expl3 which shows some nice features of the language:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\generatetable}{m}{\nrz_generate_table:n {#1}}

\seq_new:N \l_nrz_head_seq
\tl_new:N \l_nrz_preamble_tl
\tl_new:N \l_nrz_header_tl
\tl_new:N \l_nrz_body_tl

\cs_new_protected:Npn \nrz_generate_table:n #1
 {
  \seq_set_split:Nnn \l_nrz_head_seq { , } { #1 }
  \nrz_generate_table_data:
  \nrz_output_table:
 }

\cs_new_protected:Npn \nrz_generate_table_data:
 {
  \tl_set:Nn \l_nrz_preamble_tl { | c | }
  \tl_clear:N \l_nrz_header_tl
  \tl_clear:N \l_nrz_body_tl
  \seq_map_inline:Nn \l_nrz_head_seq
   {
    \tl_put_right:Nn \l_nrz_preamble_tl { c | }
    \tl_put_right:Nn \l_nrz_header_tl { & \textbf{ ##1 } }
    \tl_put_right:Nn \l_nrz_body_tl { \textbf{ ##1 } } % first col
    \seq_map_inline:Nn \l_nrz_head_seq % other cols
     {
      \tl_put_right:Nn \l_nrz_body_tl { & \use:c { origREGION ##1 destREGION ####1 } }
     }
    \tl_put_right:Nn \l_nrz_body_tl { \\ }
   }
  \tl_put_right:Nn \l_nrz_header_tl { \\ }
 }
\cs_new_protected:Npn \nrz_output_table:
 {
  \begin{tabular}{ \tl_use:N \l_nrz_preamble_tl }
  \hline
  \tl_use:N \l_nrz_header_tl
  \hline
  \tl_use:N \l_nrz_body_tl
  \hline
  \end{tabular}
 }
\ExplSyntaxOff

\begin{document}

\def\origREGIONONEdestREGIONONE{1/1}
\def\origREGIONONEdestREGIONTWO{1/2}
\def\origREGIONONEdestREGIONTHREE{1/3}

\def\origREGIONTWOdestREGIONONE{2/1}
\def\origREGIONTWOdestREGIONTWO{2/2}
\def\origREGIONTWOdestREGIONTHREE{2/3}

\def\origREGIONTHREEdestREGIONONE{3/1}
\def\origREGIONTHREEdestREGIONTWO{3/2}
\def\origREGIONTHREEdestREGIONTHREE{3/3}

\generatetable{ONE,TWO,THREE}

\end{document}

The body of the table is built with a mapping function inside another mapping function, using the same sequence for the mapping!

First of all we build the sequence from the list passed as argument to \generatetable; then we proceed to use the sequence so obtained to build the table preamble (starting with |c| and adding c| at each step), the table header (adding & <string> at each step and a final \\) and the table body, one row at a time. Rows are built similarly, with an inner loop.