On return/upon return/at return

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.

The meaning is the same, but all 3 suffer from unnaturalness. These are OK:
On their return / upon their return / at their return.

Thanks a lot!