A resistor of resistance $R$ and three identical cells of emf $E$ and internal resistance $r$ are connected as shown - AQA - A-Level Physics - Question 28 - 2020 - Paper 1

The total internal resistance, $R_{total}$, of the three cells in series is:

$R_{total} = 3r$

The total resistance in the circuit combines the external resistor and the total internal resistance:

$R_{circuit} = R + 3r$

Using Ohm's law, the current $I$ in the circuit can be found by:

$I = \frac{V_{total}}{R_{circuit}} = \frac{3E}{R + 3r}$