Calculated column that combines all values from three columns into one with break line.

ard-qua Which version of SharePoint are you using?

In past, HTML was working with the SharePoint calculated column formulas. But, it does not work anymore. Previously we were able to do it using simple formula like: 

=CONCATENATE("AAA","<br/>","BBB","<br/>","CCC")

This unsupported use of HTML in calculated column formula is now deprecated by Microsoft. You can find more information at: Handling HTML markup in SharePoint calculated fields 

So, if you just want the new lines for display purpose in list view, use JSON formatting as suggested by Rob_Elliott.

If you need to store the actual new line value in column and use it later somewhere else, you will have to use Power automate or Power Apps to update the field value with new lines whenever new item is created or updated in the list

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