Realize 3rd order Butterworth TF with R2 = 2 Ω

Method. Find $t(s)$, then $|\rho|^2 = 1-|t|^2$, then $Z_1 = R_1\dfrac{1+\rho}{1-\rho}$, then expand $Z_1$ by continued fraction. Here $t(s) = 2\sqrt{R_1/R_2}\,V_2/V_1$, and the given $T(s) = 1/(s^3+2s^2+2s+1)$ fixes the shape; its dc level is set by the terminations.

Step 1: Transmission coefficient. Third order Butterworth (Table 1): $B(s) = (s+1)(s^2+s+1) = s^3+2s^2+2s+1$. At dc the inductors are shorts and capacitors open, so the ladder reduces to a divider: $V_2/V_1 = \frac{R_2}{R_1+R_2} = \frac{2}{3} = 0.6667$. Hence the largest possible dc transmission is

$$|t(0)|^2 = \frac{4R_1R_2}{(R_1+R_2)^2} = \frac{4(1)(2)}{(1+2)^2} = \frac{8}{9} = 0.8889$$

so $|t(j\omega)|^2 = \dfrac{0.8889}{1+\omega^6}$, i.e. $t(s) = \dfrac{0.9428}{s^3+2s^2+2s+1}$.

Step 2: Reflection coefficient.

$$|\rho(j\omega)|^2 = 1 - \frac{0.8889}{1+\omega^6} = \frac{0.1111 + \omega^6}{1+\omega^6} \;\Rightarrow\; \rho(s)\rho(-s) = \frac{0.1111 - s^6}{1 - s^6}$$

The zeros satisfy $s^6 = 0.1111$, i.e. they lie on a circle of radius $a = (0.1111)^{1/6} = 0.6934$ at the Butterworth angles. Choosing the left-half-plane zeros (like the Butterworth poles, scaled by $a$):

$$\rho(s) = \frac{(s+a)(s^2+as+a^2)}{B(s)} = \frac{s^3 + 1.3867 s^2 + 0.9615 s + 0.3333}{s^3+2s^2+2s+1}$$

Step 3: Input impedance ($R_1 = 1$):

$$Z_1(s) = R_1\frac{1+\rho(s)}{1-\rho(s)} = \frac{2s^3+3.3867s^2+2.9615s+1.3333}{0.6133s^2+1.0385s+0.6667}$$

Step 4: Continued fraction expansion (computed by repeated long division):

$$Z_1(s) = 3.261\,s + \cfrac{1}{0.7789\,s + \cfrac{1}{1.181\,s + 2}}$$

So series $L_1 = 3.261$ H, shunt $C_2 = 0.7789$ F, series $L_3 = 1.181$ H, and the remainder is the load $R_2 = 2\ \Omega$, as required (this choice of $\rho$ sign gives the correct $R_2$; the other sign ends in $1/2\ \Omega$ and is rejected).

Step 5: Circuit.

  R1=1  L1=3.261 H     L3=1.181 H
 o-/\/\---UUU----+----UUU---+
 |               |          |
(~) V1     C2=0.7789 F   R2=2
 |               |          |
 o---------------+----------+

Check: analysing this ladder gives $V_2/V_1 = \dfrac{0.6667}{s^3+2s^2+2s+1}$, the required Butterworth response.

Answer: $R_1 = 1\ \Omega$, $L_1 = 3.261$ H, $C_2 = 0.7789$ F, $L_3 = 1.181$ H, $R_2 = 2\ \Omega$ (normalized to $\omega_c = 1$ rad/s).