function one(){
    echo "one";
}

function two(){
    echo "two";
}

function three(){
    echo "three";
}


function run_some_function(){
    if(!isset($_REQUEST['function_name'])) return;
    
    //Call it automatically for valid function names
    call_user_func($_REQUEST['function_name']);

    //or use if/select etc
    if($_REQUEST['function_name'] == 'one') one();
}

[add_action](http://codex.wordpress.org/Function_Reference/add_action)( '[init](http://codex.wordpress.org/Plugin_API/Action_Reference/init)', 'run_some_function');