A food truck logs daily sales and wants a smoothed trend line: a trailing three-day moving average.
daily
day is the day number since launch. Day 3 is missing — the truck was closed — so the recorded days are 1, 2, 4, 5.
Your task: return day, sales, and a moving_avg — the average sales over the current row and the two rows before it in day order (a trailing three-row window, counted by rows, not by calendar days). Early rows average over however many rows exist: day 1 is just itself, day 2 averages two.
Assign your answer DataFrame to result. Row order doesn't matter.