Are you allowed to make a U-turn at the intersection in this situation?

Why?

The green directional symbol indicates both a left turn and a U-turn. You may therefore make a U-turn because the displayed signal explicitly permits that manoeuvre. An S-3 signal showing only a left arrow would not permit a U-turn.

§ 97 ust. 1 i 2 rozporządzenia w sprawie znaków i sygnałów drogowych