Assign current directions and label loops.

Let's assume the following current directions:

• $I_1$: Current flowing upwards through the left branch (through $E_1$ and $R_1$).
• $I_2$: Current flowing upwards through the middle branch (through $E_2$ and $R_2$).
• $I_3$: Current flowing downwards through the right branch (through $R_3$).

Let's label the top junction as A and the bottom junction as B.

At junction A, currents $I_1$ and $I_2$ flow into the junction, and $I_3$