Create Index Column on Power Query based on 2 or 3 Columns

IN Power Query, first sort your data to make sure it's in a format similar to what you've shown.

Then do a 'Group BY' using the key columns (

) - you might get away with the first 3 if it's enough to show correct groups -

with one aggregation on All rows called 'all'.

Then add a custom column with this:

Table.AddIndexColumn([all], "CountInd",1,1)

This adds an index per Grouping.

You can then expand column headings to return the appropriate data.

A fair bit to get your head around but that should work.

Step 1
In Power Query, do...

Group by : Bill+Ship+Serial

New Column Name : Table_1

Operation : All Raws

Step 2

Add Custom Column...

Table_2 = Table.AddIndexColumn([Table_1],"Index",1)

Step 3

Expand Table_2

That's it.

Infact, don't need that if data is sorted by Invoice number.

Paste this in advanced editor:

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Invoice #", type text}, {"Invoice Date", type datetime}, {"Bill Customer ID", type text}, {"Ship Customer ID", type text}, {"Serial Number", type text}, {"Bill+Ship+Serial", type text}, {"Index", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Bill+Ship+Serial"}, {{"Count", each _, type table}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "IndexUse", each Table.AddIndexColumn([Count],"IndexNow",1)),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Count"}),
    #"Expanded IndexUse" = Table.ExpandTableColumn(#"Removed Columns", "IndexUse", {"Invoice #", "Invoice Date", "Bill Customer ID", "Ship Customer ID", "Serial Number", "Bill+Ship+Serial", "Index", "IndexNow"}, {"Invoice #", "Invoice Date", "Bill Customer ID", "Ship Customer ID", "Serial Number", "Bill+Ship+Serial.1", "Index", "IndexNow"})
in
    #"Expanded IndexUse"

Table1 is your sourcedata table name.

Hope it helps.