On return/upon return/at return

English as a Second Language English Vocabulary, Grammar and Idioms
1

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.

2

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

3

Thanks a lot!