Predict, then run it

levels(day, ml) = 05-01→10, 05-02→20, 05-05→5. A '3-day rolling sum' via ROWS:

SELECT day, SUM(ml) OVER (ORDER BY day ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) AS roll FROM levels ORDER BY day;

Predict the output — mind the gap before 05-05.