.add()
does exactly what you’re after.
h3.add(btn).hide();
If you wanted to make it a little more convenient for yourself, with a “merge” function like in your question, this could be added easily:
$.merge = function(objs) {
var ret = objs.shift();
while (objs.length) {
ret.add(objs.shift());
}
return ret;
};
$.merge([h3, btn]).hide()