The electric field due to C will be in the direction of line joining O and C.
Net electric Field due to B and C along OC $$= \dfrac{k2q}{d^2} + \dfrac{k2q}{d^2} = \dfrac{k4q}{d^2}$$
Now, Electric field due to A along OA $$= \dfrac{k4q}{d^2}$$
Net Electric field along the x-axis is [Ref. image 2]
$$2 \left[\dfrac{k4q}{d^2}\cos 30^o\right] = \dfrac{k4\sqrt{3}q}{d^2}$$
$$= \dfrac{\sqrt{3}q}{\pi \in_0d^2}$$