if the diameter of the cylinder is 12 inches, then the radius is half that, or 6.
now, the flavors are on a ratio of 3:2:1, that simply means, we can just get the volume of the cylinder in total, and divide it by (3+2+1), and the vanilla flavor gets 3 of that quotient.
[tex]\bf \textit{volume of a cylinder}\\\\
V=\pi r^2 h~~
\begin{cases}
r=radius\\
h=height\\
-----\\
h=18\\
r=6
\end{cases}\implies V=\pi (6^2)(18)\implies V=648\pi [/tex]
[tex]\bf \stackrel{vanilla}{3}~~:~~\stackrel{chocolate}{2}~~:~~\stackrel{strawberry}{1}
\\\\\\
\stackrel{vanilla}{3\cdot \frac{648\pi }{3+2+1}}~~:~~\stackrel{chocolate}{2\cdot \frac{648\pi }{3+2+1}}~~:~~\stackrel{strawberry}{1\cdot \frac{648\pi }{3+2+1}}
\\\\\\
\stackrel{vanilla}{324\pi }~~:~~\stackrel{chocolate}{216\pi }~~:~~\stackrel{strawberry}{108\pi }[/tex]
