A co třeba takhle?

var divs = document.querySelectorAll('nav>div');
var str ='';
for (var i = 0; i < divs.length; i++) {
    var innerDivs = divs[i].querySelectorAll('div');
    for (var j = 0; j < 3; j++) {
        var div = innerDivs[j];
        str += div.textContent;
    }
}
document.querySelector('article').innerHTML = str;