Either of these 2 should help you:
First, result in blank value if Regional Manager is not provided.
IF(ISBLANK([Test column1]),IF(ISBLANK([Test Column2]),IF(ISBLANK([Test Column3]),"",[Test Column3]),[Test Column2]),[Test column1])
Another one:
IF(ISBLANK([Test column1]),IF(ISBLANK([Test Column2]),IF(ISBLANK([Test Column3]),"",[Test Column3]),[Test Column2]),[Test column1])
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