Unpivot columns in groups of three columns in PowerQuery

Question

How can I accomplish this using PowerQuery? Data in on a worksheet.

Source table:

Col1	Col2	Att1	Att2	Att3	Att4	Att5	Att6
AAA	DDD	X	O	alpha	delta	100	400
BBB	EEE	Y	P	beta	vega	200	500

Desired table:

Col1	Col2	CustomName1	CustomName3	CustomName3
AAA	DDD	X	alpha	100
AAA	DDD	O	delta	400
BBB	EEE	Y	beta	200
BBB	EEE	P	vega	500

Answer 1

Agnostic to column names and number of columns, pulling alternate columns

let Source =Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
Leading = 2, Base=List.FirstN(Table.ColumnNames(Source),Leading),
Part1= Table.RemoveFirstN(Table.DemoteHeaders(Table.SelectColumns(Source,Base&List.Alternate(List.Difference(Table.ColumnNames(Source),Base),1,1))),1),
Part2= Table.RemoveFirstN(Table.DemoteHeaders(Table.SelectColumns(Source,Base&List.Alternate(List.RemoveFirstN(Table.ColumnNames(Source),Leading),1,1,1))),1)
in Part1 & Part2

If you need to, first add an leading index column, and change Leading=3, resort at end

EDIT to provide a final rename

let Source =Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
Leading = 2, Base=List.FirstN(Table.ColumnNames(Source),Leading),
Part1= Table.RemoveFirstN(Table.DemoteHeaders(Table.SelectColumns(Source,Base&List.Alternate(List.Difference(Table.ColumnNames(Source),Base),1,1))),1),
Part2= Table.RemoveFirstN(Table.DemoteHeaders(Table.SelectColumns(Source,Base&List.Alternate(List.RemoveFirstN(Table.ColumnNames(Source),Leading),1,1,1))),1),
NewNames= List.FirstN(Table.ColumnNames(Source),Leading)&List.Transform({1 .. List.Count(List.RemoveFirstN(Table.ColumnNames(Part1),Leading))}, each "Custom Name"&Text.From(_)),
#"Rename"=Table.RenameColumns( Part1 & Part2,List.Zip({Table.ColumnNames(Part1),NewNames}))
in #"Rename"