Your mistakes is here `main_profile` , profile type always start with **profile_name** . just need change main_profile to **profile_main**

    global $user;  
    $user->uid ;
    print_r($user->uid);
    print "print out the user value" . $user->uid;
    
    $uid =3;
    
    $account = user_load($user->uid); 
    $profile = profile2_load_by_user($account);
    
    
    $print_first_name =  drupal_render(field_view_field('profile2',$profile['profile_main'], 'field_first_name'));
    print $print_first_name;

If you want see all your profile type and correct name and it's contents print_r or var_dump `$profile`

    $profile = profile2_load_by_user($account);
    print_r($profile);