As a plugin:
jQuery.fn.wrapInChunks = function(html, chunkSize) {
chunkSize = chunkSize || 1;
var items = this.get(),
rows = [],
cur = rows[0] = $(html);
while (items[0]) {
if (rows[rows.length - 1].children().length === chunkSize) {
cur = rows[rows.length] = $(html);
}
cur.append( items.shift() );
}
return this.pushStack(rows);
};
$('.boxgrid').wrapInChunks('', 3).appendTo('.items');