Generally, the solution is to not use fminsearch. Use a better optimizer instead. It will be faster, and you will get a better answer. What can I say?
FMINCON would be my first recommendation. However, other tools, like LSQLIN or INTLINPROG might be appropriate, since I don't know your actual problem.
CAN you use fminsearch? Well, yes. You could do so. If I were pressed, I would start by replacing c completely in your problem, replacing it with 1-a-b. Then I would use a penalty function approach. Start out with an initial value for fminsearch that satisfies your requirements, that a+b <= 1. INSIDE your objective function, IF fminsearch passes in a set of parameters where a+b is NOT acceptable, then don't even bother trying to evaluate the function, just return inf. Fminsearch will not be fazed by this infinite penalty on the objective, and it should produce a result that satisfies your goals as long as you start with a feasible point.
If you want a better, more complete answer, then I would need to see more information. What is your ACTUAL problem? Perhaps this is a simple linear problem in some way, and a tool such as intlinprog or lsqlin would be sufficient.
