Merging jQuery objects

.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()

$.map can flatten arrays:

function merge(array_of_jquery_objects) {
    return $($.map(array_of_jquery_objects, function(el) {
        return el.get();
    }));
}

$(btn).add(h3).hide();

Not sure if it works though, documentation for add doesn't mention haivng the jQuery object as a parameter but only a list of elements, so if that doesn't work this should:

$(btn).add(h3.get()).hide();