Let \( \tan\alpha, \tan\beta \) and \( \tan\gamma \,;\ \alpha,\beta,\gamma \neq \dfrac{(2n-1)\pi}{2},\ n\in \mathbb{N} \) be the slopes of three line segments \( OA \), \( OB \) and \( OC \), respectively, where \( O \) is origin. If circumcentre of \( \triangle ABC \) coincides with origin and its orthocentre lies on \( y \)-axis, then the value of \( \left(\dfrac{\cos 3\alpha+\cos 3\beta+\cos 3\gamma}{\cos\alpha\cos\beta\cos\gamma}\right)^{2} \) is equal to ________.