Transform Multiple columns by multiplying by another column

You can't access other columns from with Table.TransformColumns, so you have to use Table.TransformRows.  Here's one way to do it in the query editor.  To see how it works, just create a blank query, open the Advanced Editor and replace the text there with the M code below.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMrTQMzTQMzIwMlTSUTI0MjYBUs75Ral+ibmpYCGosC6IYQTjgBgmMI4xkGGmFKuDaZgZkmEgLaZQYbBhxjAOiowxmIPNMAskw8B6DaDiukjutACbZoTMMVaKjQUA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date = _t, SKUbundle = _t, Name = _t, Quantity = _t, SKU1 = _t, Quantity1 = _t, SKU2 = _t, Quantity2 = _t, SKU3 = _t, Quantity3 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type text}, {"SKUbundle", Int64.Type}, {"Name", type text}, {"Quantity", Int64.Type}, {"SKU1", type text}, {"Quantity1", Int64.Type}, {"SKU2", type text}, {"Quantity2", Int64.Type}, {"SKU3", type text}, {"Quantity3", Int64.Type}}),
    Custom1 = Table.FromRecords(Table.TransformRows(#"Changed Type", (x) => Record.TransformFields(x, List.Transform({"Quantity1", "Quantity2", "Quantity3"}, (y) => {y, each x[Quantity] * _}))))
in
    Custom1

Pat