Try playing around with ExecuteInEditMode.

So like this:

@script ExecuteInEditMode()

var a;
var b;
var c;

void Update()
{
    c = a + b;
}

(I haven't tested this.)

As unity does not display properties with getters/setters, your best bet would be writing a custom inspector.

yep, this is working! thanks!

i just added the var types for a,b,c, i get some errors if not... here is the javascript version:

@script ExecuteInEditMode()

var a:float;
var b:float;
var c:float;

function Update() {
    c = a + b;
}