Edit in JSFiddle

var matA = [1, 0, 0,
            0, 1, 0,
            1, 2, 1];

var matB = [1, 0, 0,
            0, 1, 0,
            3, 4, 1];

var out = [];

mat3.multiply(out, matA, matB);

document.getElementById('result').innerHTML = out;

<div id="result"></div>