Edit in JSFiddle

$(document).ready(function () {
    // Load the first 3 list items from another HTML file
    //$('#myList').load('externalList.html li:lt(3)');
    $('#myList li:lt(3)').show();
    $('#showLess').hide();
    var items =  25;
    var shown =  3;
    $('#loadMore').click(function () {
        $('#showLess').show();
        shown = $('#myList li:visible').size()+5;
        if(shown< items) {$('#myList li:lt('+shown+')').slideDown();}
        else {$('#myList li:lt('+items+')').slideDown();
             $('#loadMore').slideUp();
             }
    });
    $('#showLess').click(function () {
        $('#myList li').not(':lt(3)').slideUp();
    });
});

<ul id="myList">
    <li>One</li>
    <li>Two</li>
    <li>Three</li>
    <li>Four</li>
    <li>Five</li>
    <li>Six</li>
    <li>Seven</li>
    <li>Eight</li>
    <li>Nine</li>
    <li>Ten</li>
    <li>Eleven</li>
    <li>Twelve</li>
    <li>Thirteen</li>
    <li>Fourteen</li>
    <li>Fifteen</li>
    <li>Sixteen</li>
    <li>Seventeen</li>
    <li>Eighteen</li>
    <li>Nineteen</li>
    <li>Twenty one</li>
    <li>Twenty two</li>
    <li>Twenty three</li>
    <li>Twenty four</li>
    <li>Twenty five</li>
</ul>
<div id="loadMore">Load more</div>
<div id="showLess">Show less</div>

#myList li{ display:none;
}
#loadMore {
    color:green;
    cursor:pointer;
}
#loadMore:hover {
    color:black;
}
#showLess {
    color:red;
    cursor:pointer;
}
#showLess:hover {
    color:black;
}