I believe the reAuth plugin makes the spoofing obsolete, and technically spoofing the FW makes the reAuth plugin obsolete.
The auto update setting is still needed anyway as there could always be a newer FW release than the one you're currently spoofing. If the PSN store expects you to be on 3.74 you have to spoof to that specific version, you can't just spoof to 3.75 or higher, because that will also fail. But that also means if Sony releases a 3.75 update your auto update might trigger because it thinks you're "only" on 3.74.
All in all these are options that exist for a specific purpose, but not every purpose might be relevant to your setup specifically, for example because you're using reAuth.
Edit: I'd recommend just letting all 3 active unless you encounter issues. Best to cover all your bases. But I'm not an expert on the matter as my own Vita is patched further ahead than yours.