I am experiencing this as well: https://developer.apple.com/forums/thread/712547

The link for watchOS 8.7.1 Series 3 device support is here: https://devimages-cdn.apple.com/downloads/xcode/dsc/Watch3_3_19U67.dmg. When I try to access it in Safari, I get the "access denied" error. When I try to download the watchOS 8.0 Series 3 device support at https://devimages-cdn.apple.com/downloads/xcode/dsc/Watch3_3_19R346.dmg, I can download this no problem. This seems like an error with Apple's cdn permissions?