If the fields of the structure have no meaning and only their order is important, then you shouldn't be using structures. If all you're storing in your fields are scalars, then a matrix would be better, otherwise a cell array. Using structure fields for that is just as bad as using dynamically generated variable names.
So, assuming you're not using the structure as a structure, I'd convert it to a cell array with struct2cell and then optionally to a matrix with cell2mat:
c = struct2cell(a);
%accessing the 3rd variable is then:
var3 = c{3};
%if all values are scalar, then:
m = cell2mat(c);
var3 = m(3);
fn = fieldnames(a);
var3 = a.(fn{3});
