the distance 's' that an object falls is proportional to the square of the time 't' of the fall thus we can write

s = k*t^2, where k is constant

lets find k

for t = 1s we have s = 16 ft that is

16 = k*1^2

16 = k*1

k = 16

thus the formula for the distance is s = 16*t^2

How far will it fall in 3 seconds?

for t = 3s

s = 16*3^2

s = 144 ft

in 3 second it will fall 144 feet.

How long will it take an object to fall 400 feet?

s = 400 ft and we need to find t

400 = 16*t^2

t = (400/16)^0.5

t = 5s

it takes 5 seconds for the object to fall 400 feet.