I'm not sure why you're using a loop to create the same plots three times with different symbols. Are there other columns (of a,b,c etc) that need to be plotted as the loop increments?
Alternatively, do you mean to simply plot each of these three things once, but automatically using a unique symbol for each?
If that's the case, and your data doesn't lend itself to indexing, then you might consider turning symbol into a function. That way you can just do something like this:
symbolList = ['o', 'x', 's', 'd','.', '^', 'v', '>', '<', '*','p','h','+'];
symbol = @(k) sprintf('r%s-', symbolList(mod(k-1,length(symbolList))+1));
plot((1:10)+5, symbol(1), 'LineWidth', 0.1);
plot((1:10)+10, symbol(2), 'LineWidth', 0.1);
plot((1:10)+15, symbol(3), 'LineWidth', 0.1);
Obviously I'm just using placeholder data here.