# the function

def divisibleby(value, arg):
    "Returns true if the value is devisible by the argument"
    return int(value) % int(arg) == 0


# template

    {% for athlete in athletes_team.list %}
    {% if forloop.counter|divisibleby:"3" %}<tr>{% endif %}
        <td{% if athlete.chosen %}class="selected"{% endif %}>
            <input type="checkbox" name="id" id="athlete{{ athlete.id_ }}" value="{{ athlete.id_ }}" {% if athlete.chosen %}checked{% endif %}>
            <label for="athlete{{ athlete.id_ }}">{{ athlete }}</label>
        </td>
    {% if footloop.counter|add:"2"|divisibleby:"3" %}</tr>{% endif %}
    {% endfor %}