On return/upon return/at return

[PredatorTheSkullCollector]

Good evening fellows,
On several websites I've seen different usages of the following word "return"
For example:
On return/upon return/at return
So,I've been wondering is the meaning of the three examples similar?Or maybe it depends on the context..?
I'm sorry if my asking isn't appropriate.

[Mister Micawber]

The meaning is the same, but all 3 suffer from unnaturalness. These are OK:
On their return / upon their return / at their return.
_________________
Native English teacher at Mister Micawber's

[PredatorTheSkullCollector]

Thanks a lot!