If we want that minimum three (more than two) ball remain in air then time of flight of first ball must be greater than $4\, sec$.

$T > 4\,sec$

$\frac{{2u}}{g} > 4\;sec \Rightarrow u > 19.6\;m/s$

for $u =19.6$. First ball will just strike the ground(in sky)

Second ball will be at highest point (in sky)

Third ball will be at point of projection or at ground (not in sky)