Not a BETA 3 but, but a area51.phpbb.com bug that could be still in BETA 3, will check it out as soon as possible.

[phpBB Debug] PHP Notice: in file /viewtopic.php on line 1321: Undefined index: username
[phpBB Debug] PHP Notice: in file /viewtopic.php on line 1321: Undefined index: user_colour
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3380: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2921)
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3382: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2921)
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3383: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2921)
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3384: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2921)

in this page:

http://area51.phpbb.com/phpBB/viewtopic.php?f=4&t=25439&start=10" target="_blank

didn't try to reproduce it on a fresh BETA3, just reloaded it a few times. I think you'll know what's the problem, i'll try to look it up in the meantime.