At the beginning of the 3 sec, its speed is 10 m/s.
Over the course of 3 sec, gravity adds (9.8 x 3) = 29.4 m/s
to its speed, so its speed after 3 sec is (10 + 29.4) = 39.4 m/s.
Its average speed during the 3 sec is
(1/2) (10 + 39.4)
= (1/2) (49.4) = 24.7 m/s .
In 3 sec, at an average speed of 24.7 m/s,
the rock travels
(3) x (24.7) = 74.1 meters.
If there's no air resistance in its path, then it was thrown
from 74.1 meters above the ground.