How to use Power Query List.Max for multiple columns?

Hi Anonymous 

It would be better to pivot the columns but if you want to do it like this, paste this M code in  a blank query to see the steps. The last one is the relevant one:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTICYmOl2FgA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date1 = _t, Date2 = _t, Date3 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date1", Int64.Type}, {"Date2", Int64.Type}, {"Date3", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each List.Max(Record.ToList(_)))
in
    #"Added Custom"

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Cheers