begin{document}
$$ F_x=
quad
begin{bmatrix}
1 & 0 & 0 & 0 & ... & 0 \
0 & 1 & 0 & 0 & ... & 0 \
0 & 0 & 1 & 0 & ... & 0 \
  &   &   &   & 3N Columns
end{bmatrix}
$$
end{document}