Hello, kashmoneyrecord3!
I don't understand your game plan,
but "just sub in 3" sounds like a bad idea . . .
Use the method of cylindrical shells to find the volume generated by rotating
the region bounded by the given curves about the specified axis.
Sketch the region and a typical shell.
. . $\displaystyle y\:=\:4xx^2,\; y\,=\,3,\;\text{ about }x=1$
Did you make a sketch?
Code:

 : ..*..
 :.*:::::::*.
3+ *     *
 *: :*
 : :
* : : *
 : :
 : :
*++*
 1 3 4

Formula: .$\displaystyle V \;=\;2\pi\! \int^b_a\!\text{(radius)(height)}\,dx $
We have: .$\displaystyle \begin{Bmatrix}r \,=\, x1 \\ h \,=\,(4xx^2)  3 \\ a \,=\, 1 \\ b \,=\, 3 \end{Bmatrix}$
Hence: .$\displaystyle V \;=\;2\pi\! \int^3_1\!(x1)(4xx^23)\,dx$