There should be at https://doc.e.foundation/devices/FP3/upgrade, but just briefly browsing through it, it seems automatically generated and not right for the Fairphone 3 to me, so better follow the normal install instructions at https://doc.e.foundation/devices/FP3/install, the Android R builds are available there, too.
I haven’t done this myself yet, though.
I don’t use Magisk, so I would start with the official documentation … https://topjohnwu.github.io/Magisk/install.html … else forum searches here and in the Fairphone forum might bring up something.
It’s beta, it’s in testing, so a healthy dose of caution can’t hurt. Resources to follow proceedings …