Top 3 Lowest value's

Here is one way to do it with a DAX measure in a Table visual with your Product Number and Online Store columns

Lowest 3 Price =
VAR thisProduct =
    MIN ( 'Price'[Product Number] )
VAR lowest3 =
    TOPN (
        3,
        FILTER ( ALL ( 'Price' ), 'Price'[Product Number] = thisProduct ),
        'Price'[Price], ASC
    )
VAR result =
    CALCULATE ( SUM ( 'Price'[Price] ), KEEPFILTERS ( lowest3 ) )
RETURN
    result

Pat