Edit in JSFiddle

var a = document.getElementById("a"),
    b = document.getElementById("b"),
    c = document.getElementById("c"),
    x, y, z,
    sumar = document.getElementById("sumar"),
    resultado = document.getElementById("resultado");

sumar.addEventListener("click", function(){
    x = !isNaN(a.value) ? Number(a.value) : 0;
    y = !isNaN(b.value) ? Number(b.value) : 0;
    z = !isNaN(c.value) ? Number(c.value) : 0;
    resultado.innerHTML = x + y + z;
}, false);

<div>
    Input A: <input type = "text" id = "a" />
</div>
<div>
    Input B: <input type = "text" id = "b" />
</div>
<div>
    Input C: <input type = "text" id = "c" />
</div>

<button id = "sumar">Sumar</button>
<label id = "resultado"></label>