Was the worshiper who joined the imam during the 'Isha prayer while the latter was praying Maghrib correct in what he did, separating from the imam after the middle Tashahhud to pray a third rak'ah individually and then concluding, only to rejoin the imam to complete the 'Isha prayer in congregation?

The ruling is based on what the worshipper did concerning breaking the following (of the Imam), according to the most preponderant opinion among jurists. So, if it were permissible for the congregant to intend to break from following the Imam and complete the prayer individually, then what he did would be permissible and his prayer valid. The Fiqh Encyclopedia mentions a difference of opinion regarding the validity of the prayer when separating [from the Imam], with some holding it valid absolutely, others holding it invalid absolutely, and some differentiating between separating with an excuse and without one. Ibn Qudamah considers separation with an excuse permissible, and for separation without an excuse, there are two narrations: invalidity of the prayer or its validity. Validity is only for one who intended separation and then completed the prayer, not for one who completed it individually before intending separation. It is better for the worshipper to continue with the Imam and not break from following him without an excuse.