SO Fiddle - Checkbox

HTML

<table id="myTable">
    <tr>
        <td>
            <input type="checkbox" />
        </td>
        <td>Lorem</td>
        <td>Lorem</td>
        <td>Lorem</td>
    </tr>
    <tr>
        <td>
            <input type="checkbox" />
        </td>
        <td>Lorem</td>
        <td>Lorem</td>
        <td>Lorem</td>
    </tr>
    <tr>
        <td>
            <input type="checkbox" />
        </td>
        <td>Lorem</td>
        <td>Lorem</td>
        <td>Lorem</td>
    </tr>
    <tr>
        <td>
            <input type="checkbox" />
        </td>
        <td>Lorem</td>
        <td>Lorem</td>
        <td>Lorem</td>
    </tr>
    <tr>
        <td>
            <input type="checkbox" />
        </td>
        <td>Lorem</td>
        <td>Lorem</td>
        <td>Lorem</td>
    </tr>
    <tr>
        <td>
            <input type="checkbox" />
        </td>
        <td>Lorem</td>
        <td>Lorem</td>
        <td>Lorem</td>
    </tr>
    <tr>
        <td>
            <input type="checkbox" />
        </td>
        <td>Lorem</td>
        <td>Lorem</td>
        <td>Lorem</td>
    </tr>
</table>
<p id="hello">
    <button id="compare" type="button">Compare</button>
</p>
<p id="error">Select more than one to compare</p>

CSS

#hello, #error {
    display:none;
}
table tr td {
    border:1px solid #ddd;
}

JavaScript

$(function () {
    var tb = $("#myTable")
    tb.find("input:checkbox").filter(':has(:checkbox:checked)').end().click(function () {
        var bol = tb.find("input:checkbox:checked");
        if (bol.length <= 3) {
            $('#error').hide();
            $('#hello').fadeIn();
        } else if (bol.length >= 4) {
            tb.find("input:checkbox").not(":checked").attr("disabled", bol);
        } else {
            $('#hello').hide();
        }
        if (bol.length < 4) {
            tb.find("input:checkbox").not(":checked").attr("disabled", false);
        }
        if (bol.length <= 1) {
            $('#error').show();
            $('#compare').hide();
        }
        if (bol.length > 1) {
            $('#compare').show();
        }
    });
    $('#compare').click(function () {
        var ck = tb.find("input:checkbox:checked");
        if (ck.length === 1) {
            $('#error').show();
        }
    });
});