I have no idea what changes have taken place from the previous release of beta 3, I am going to assume small bug fixes. says Flexbeta
http://www.flexbeta.net/main/comments.php?id=21274&catid=1

source: http://www.microsoft.com/downloads/details.aspx?FamilyID=4c1a8fbe-fb6a-47ac-867d-bb1f17e477ee&DisplayLang=en

download link:
http://download.microsoft.com/download/7/8/3/783fb2b5-bfaf-4712-8557-952af02b1b8d/IE7BETA3-WindowsXP-x86-enu.exe