Thevenin Equivalent Voltage and Resistance with Multiple Parallel Sources
Many methods to solve. One simple method for this particular type of circuit is using Millman's theorem.

$$V_{th} = V_{AB} = \frac {\frac{E_1}{R_1}+\frac{E_2}{R_2}+\frac{-E_3}{R_3}}{\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}}$$

Millman's theorem

Or

Convert all voltage sources + series resistances To current sources with parallel resistances. And multiply the net current by net resistance across AB.

Or

Use superposition theorem to find \$V_{AB}\$, considering one source at a time. Finally add the \$V_{ABs}\$ you got for individual sources.

Or

Apply nodal analysis at node A and find the voltage at node A. That would be \$V_{AB}\$