Auto-fill fields

Greetings,
having not been able to solve my problem in another topic, I try to think differently to get around it.

The question is:
is it possible to fill a multi-select field dynamically using values entered in another field?

Goal: Use a single “Country” search filter, returning all users that have that value in 3 different “Country” fields.

From what I understand it is not possible to have a function do this.

Possible solution, I have three “Country” fields
Country 1: Italy ( Metakey: Country_1)
Country 2: France ( Metakey: Country_2 )
Country 3: Switzerland ( Metakey: Country_3)

Let’s assume that I had another multi-select field not visible to the user with metakey “Country_4” which fills with the values of “Country_1 + Country_2 + Country_3”, I could use the field “Country_4” for search filters in order to return the results for all 3 metakeys.

Do you think it is possible?
Thanks.