Xcode 9 beta 403 forbidden
Paid up developers can not see any old betas for redownload. You typically need a support case and a very compelling reason to get anything beta from Apple once they update to a new beta. Save your downloads if you ever think you need them later.
That being said, all the release versions of 9 that are available:

https://developer.apple.com/download/more/

Assuming you don’t want the last version of 8, here is the oldest 9.0

https://download.developer.apple.com/Developer_Tools/Xcode_9/Xcode_9.xip