Multi Column Relationships in the same table

try like this:

let

    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("XY3LCQAxCER78ZyL31qSlfTfxo6ysCGDhyc+nExiGqTC5gGo2SNJAE8HMDsAFi2tBMW6Oqfw/SnBTuH/XycHiF6NcRWFeZ/2Cw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [UID = _t, A = _t, B = _t, C = _t]),
    
    ChangedType = Table.TransformColumnTypes(Source,{{"UID", Int64.Type}, {"A", type text}, {"B", type text}, {"C", type text}}),
    
    AddIDCol = Table.AddColumn( 
                        ChangedType, 
                        "ID",
                   each if 
                            [C] <> "" 
                        then 
                            [C]
                        else
                            if 
                                [B] <> ""
                            then
                                [B]
                            else
                                [A],
                            type text
    )
                            
    
    
in
    AddIDCol