A) First we calculate the volume of each ball:

\( \boxed{V=\frac{4\pi r^3}{3}}\\ \\ \boxed{V=\frac{4*(3,14)*(1.25)^3}{3}=8,177 \ square \ inch} \)

b) Now we calculate the volume of tree balls:

\( \boxed{V_T=3*8,177=24.531 \ square \ inch} \)

c) Now we calculate the volume of cylinder:

c1) Base area:

\( \boxed{A_b=\pi r^2}\\ \\ \boxed{A_b=3,14*(1,25)^2=4.906 \ square \ inch} \)

c2) Cylinder height

\( \boxed{h=3*(2.5)=7.5 \ inch} \)

c3) Cylinder Volume:

\( V_C=A_b*h\\ \\ \boxed{V_C=4.906*7.5=36.795 \ cubic \ inch} \)

d) Finally we calculate the internal space:

\( \boxed{\boxed{s=36.795-24.531= 12,264\ cubic \ inch}} \)

\( d=2.5 \ in \\ \\ r=\frac{d}{2}=\frac{2.5}{2}=1.25 \ in \\ \\ \pi=3.14 \\ \\Volume \ of \ a \ Cylinder : \\ \\V = \pi r^2 h \)

\( h=3\cdot d = 3*2.5 = 7.5 \ in \\ \\V_{c} = 3.14\cdot (1.25)^2\cdot 7.5 = 23.55 * 1.5625=36.80 \ in^3 \)

\( The \ volume \ of \ tree \ balls : \\ \\ V_{3b}= 3 \cdot \frac{4}{3} \pi r^3 = 4\pi r^3 \\ \\V_{3b}=4\cdot 3.14\cdot (1.25)^3= 12.56\cdot 1.9531 \approx 24.53 \ in^3 \\ \\V_{c}-V_{3b}=36.80-24.53 =12.27 \ in^3 \\ \\ Answer : \ The \ volume \ not \ occupied \ by \ balls \ it \ 12.27 \ in^3 \)