We don't know anything about your table, so here is something that works on my environment

;WITH cteBaseData AS (

SELECT [Timestamp], Customer

, seq = ROW_NUMBER() OVER (PARTITION BY Customer ORDER BY [Timestamp] DESC)

FROM #Customer

)

DELETE src

FROM #Customer AS src

INNER JOIN cteBaseData AS cte

ON src.Customer = cte.Customer

AND src.[Timestamp] = cte.[Timestamp]

WHERE cte.seq > 3;