A dc source with internal resistance R 0 is connected to three identical resistors each of resistance R as…

In case of the highest power generated the value of internal resistance should be equal to a total external resistance, here all three external resistance are connected in parallel,

$$\begin{gathered} \Rightarrow R_0=\frac{R}{3} \\ \Rightarrow R=3R_0 \end{gathered}$$