begin{equation}
A =
begin{bmatrix}
pgfplotsforeachungrouped m in {1,...,3}
{
  pgfplotsforeachungrouped n in {1,...,3}
  {
    a_{m,n} &
  } \
}
end{bmatrix}
end{equation}

documentclass{article}
usepackage{amsmath,bm}
usepackage{xparse}

ExplSyntaxOn

NewDocumentCommand{symbolicmatrix}{O{#2}mm}
 {% #1 = optional number of rows (default square)
  % #2 = number of columns
  % #3 = symbol to repeat
  scumcoder_matrix:nnn { #1 } { #2 } { #3 }
 }

tl_new:N l__scumcoder_matrix_body_tl

cs_new_protected:Nn scumcoder_matrix:nnn
 {
  tl_clear:N l__scumcoder_matrix_body_tl
  int_step_inline:nnnn { 1 } { 1 } { #1 }
   {
    int_step_inline:nnnn { 1 } { 1 } { #2 - 1 }
     {
      tl_put_right:Nn l__scumcoder_matrix_body_tl { #3sb{##1 ####1} & }
     }
    tl_put_right:Nn l__scumcoder_matrix_body_tl { #3sb{##1 #2} \ }
   }
  begin{bmatrix}
  tl_use:N l__scumcoder_matrix_body_tl
  end{bmatrix}
 }

ExplSyntaxOff

begin{document}

[
frac{d}{dt}
begin{bmatrix}
bm{varepsilon} \
dot{bm{varepsilon}} \
bm{rho}
end{bmatrix}
=
symbolicmatrix{3}{mathbf{F}}
begin{bmatrix}
bm{varepsilon} \
dot{bm{varepsilon}} \
bm{rho}
end{bmatrix}
]

[
symbolicmatrix[2]{4}{a}
]

end{document}