9 is the square of 3. Square root is the inverse operation of square, which results in an integer. \sqrt{9} = 3. So it's not an irrational number.

3\sqrt{8}=3\sqrt{2\cdot 4 }=6\sqrt{2}

Simplifying the number gives a number with square root of a prime number. So it is an irrational number.

-\sqrt{100}=-10

100 is also a perfect square. Square root of 100 gives an integer 10

\sqrt{0.64} =0.8

Square root of a decimal which is derived from an perfect square may result in a decimal, for example, 0.8 in this case. So square root of 0.64 is a rational number.