Step 1: Understanding the situation.
In an equilateral triangle, when three forces \( F_1, F_2, F_3 \) are acting along the sides, the total torque at the center 'O' should be zero for equilibrium.
Step 2: Balancing the forces.
For the torques to balance and result in zero net torque, the forces must be arranged such that the sum of the forces \( F_3 \) equals the sum of \( F_1 \) and \( F_2 \). Thus, the magnitude of \( F_3 \) is \( F_1 + F_2 \).
Step 3: Conclusion.
The correct answer is \( F_3 = F_1 + F_2 \), which balances the torques and results in no net torque at the center.