I’m not where I can I can test this at the moment, but in theory if you have the automation plug-in there is a workaround.
There is a script that lets you send a PM based on a trigger, and being added to a group is one of the possible triggers. In theory, setting the group to trust_level_3 should work.
We use that script pretty heavily on my RP forums to send PMs when we add users to our NPC_Managers group.
They said, I actually think it’s a bit odd there isn’t a function for this in core for TL3.
I can get not having one for TL4 since that is manual, but TL3 should definitely have one for consistency 