To find out how many square feet Ivan can paint in 3 hours, we first determine how many half-hour intervals are in 3 hours.
3 hours = 3 x 2 = 6 half-hour intervals.
Since Ivan can paint 25 square feet every half hour, we multiply the number of half-hour intervals by the amount he can paint in each interval:
6 half-hours × 25 square feet/half-hour = 150 square feet.
So, the answer is 150 square feet.