There isn’t a signal because PathFollow3D doesn’t do anything on its own to move a Node. It just finds a point on the parent Path3D based on it’s progress/progress_ratio value.
I would suggest wherever you are updating the PathFollow3D’s progress, add a check for:
<span style="color:#323232;">if path_follow.progress_ratio >= 1.0:
</span><span style="color:#323232;"> # do something here, like emit a custom signal
</span>