Astone is dropped from the top of a tower. what is its velocity after 3.0 seconds?

For purposes of completing our calculations, we're going to assume that
the experiment takes place on or near the surface of the Earth. 

The acceleration of gravity on Earth is about 9.8 m/s², directed toward the
center of the planet.  That means that the downward speed of a falling object
increases by 9.8 m/s for every second that it falls.

3 seconds after being dropped, a stone is falling at (3 x 9.8) = 29.4 m/s. 

That's the vertical component of its velocity.  The horizontal component is
the same as it was at the instant of the drop, provided there is no horizontal
force on the stone during its fall.