The equivalent circuits is
Applying KVL, we get
\begin{gathered} V-\frac{I}{3} R-\frac{I}{6} R-\frac{I}{3} R=0 \\ V=I\left\lgroup \frac{R}{3}+\frac{R}{6}+\frac{R}{3} \right\rgroup=I \frac{5 R}{6} \\ \Rightarrow \frac{V}{I}=R_{ eq }=\frac{5}{6} \Omega \end{gathered}