Same issue with Xcode 16 beta 3, and iOS 18 beta 3 download.
I was able to download directly:
https://developer.apple.com/download/all/
and add to Xcode via terminal:
xcode-select -s /Applications/Xcode-beta.app
xcodebuild -runFirstLaunch
xcrun simctl runtime add "~/Downloads/iOS_18_beta_3_Simulator_Runtime.dmg"