Astone was thrown vertically downwards from the top of a tower. if the initial velocity was 3.0 m/s, calculate the velocity with which the stone hit the ground 2 seconds later?

You haven't specified where the experiment took place.  I'm going to
assume that it was somewhere on Earth.  That's important.

The acceleration of gravity on or near the surface of the Earth is about 9.8 m/s² . 
That means that a falling object falls 9.8 m/s faster every second.

So in 2 seconds of falling, its downward speed increases (2 x 9.8) = 19.6 m/s .

If the stone was falling at the rate of 3.0 m/s when the 2 seconds began,
then its downward speed at the end of the 2 seconds was

                   (3.0 m/s) + (19.6 m/s)  =  22.6 m/s