Encuentra registros con 3 o más registros consecutivos con el mismo valor

Puede usar un truco para enumerar las transacciones de "efectivo". Este truco es una diferencia de números de fila y es muy útil:

select t.*
from (select t.*, count(*) over (partition by grp, customerid, transtype) as cnt
      from (select t.*,
                   (row_number() over (partition by customerid order by date) -
                    row_number() over (partition by customerid, transtype order by date)
                   ) as grp
            from t
           ) t
      where transtype = 'cash'
     ) t
where cnt >= 3;

Esto devuelve los clientes y la fecha de inicio. Si desea devolver las transacciones reales, puede usar un nivel adicional de funciones de ventana:

select customerid, min(date) as start_date, sum(value) as sumvalue
from (select t.*,
             (row_number() over (partition by customerid order by date) -
              row_number() over (partition by customerid, transtype order by date)
             ) as grp
      from t
     ) t
where transtype = 'cash'
group by grp, transtype, customerid
having count(*) >= 3;