Why, if you already have other inequality constraints, is this a question? And how you wrote it, using A is inconsistent with you having only 3 variables. So, clearly you have more variables in your real problem, and you even already have more than one linear inequality constraint!
If you had only one constraint, and you had only 3 variables, in the order b, a, and c, you would write:
A = [-1/2, 1, 1/2];
b = 0;
But, assuming this is the THIRD linear inequality constraint, AND that the variables concerned are variables 4, 2, and 5 in the set, you would do this:
A(3,[2 4 5]) = [-1/2, 1, 1/2];
b(3) = 0;
Note that no matter what you do, you cannot enforce a STRICT inequality with < as opposed to <=. Thus you can only set a constraint as:
a - b/2 + c/2 <= 0
and even that is only enforced to within a constraint tolerance.