Do you mean also
$b\ge0$ and
$c\ge0$? Else, for each
$a$ there are an infinite number of integer pairs
$a,b$.
Otherwise you could generate an index set with:
t = Flatten[Table[{i, j, k}, {i, 0, 33}, {j, 0, 33}, {k, 0, 33}], 2];
indx=Select[t, (Total[#] === 33) \[And] (#[[2]] + #[[3]] > 0) &];
