O is the centre of an equilateral triangle ABC $F_1, F_2, F_3$ are three forces acting along the sides AB, BC and AC as shown in fig.What should be the magnitude of $F_3$ so that total torque about O is zero :

Question

$\begin{array}{1 1} (1)\; |\overrightarrow{F_3}| = |\overrightarrow{F_1}|+|\overrightarrow{F_2}| \\ (2)\; |\overrightarrow{F_3}| = |\overrightarrow{F_1}| - |\overrightarrow{F_2}| \\ (3)\; |\overrightarrow{F_3}| = \overrightarrow{F_1}+2\overrightarrow{F_2} \\ (d)\; Not \; possible \end{array}$