Find the volume of the can and subtract the volume of the balls to find the free space.
Using 3.14 for pi:
We know that the area of a cylinder (can) is

.
Our diameter is 2.7, so our radius will be half of that, 1.35.
Our height is 8.5.
Let's find the volume.

Now, we know that the volume of a sphere (tennis ball) is

.
Our diameter is 2.5, so our radius will be half of that, 1.25.
Let's find our volume.

We have three tennis balls, and 3 times that is 24.53125.
Now we subtract the 3 spheres' volumes from our cylinder's volume to find the empty space.
36.0315 - 24.53125 =
11.50025 in³