Power Query Table Select Rows

Hi,

Workbook with attempts attached.

I've been trying to sum the last 3 values based on dates for each customer, , but rather than grouping I'd like to use Table.SelectRows;

I've got

 Table.AddColumn(#"Changed Type", "Custom", each Table.SelectRows( Source, (A)=> A  [Customer] = _[Customer] ) )

This returns a column of tables of each customer, withing this I'd like to restrict this to only the last / max 3 dates.

I've got as far as

 Table.AddColumn(#"Changed Type", "Custom", each Table.SelectRows( Source, (A)=> A  [Customer] = _[Customer] and
             
                                                                                                                                  A[Date] >=  List.Min( List.MaxN( Source  [Date] , 3 ) ) ))

but the last clause is seeing all the dates, I did think of using

A[Date] >=  List.Min( List.MaxN( A [Date] , 3 ) ) ))

or A Source [Date] ,
but no good,
Any Suggestions,

Richard