Analysis of information sources in references of the Wikipedia article "Avatar" in English language version.
Some say that Krishna is Vishnu, Balarama is Shiva and Subhadra is Devi, thus the three siblings represent the three main schools of Hindu theism: Vaishnava, Shaiva and Shakta.
So Balarama became (Bala+Deva) Baladeva. Krishna and Balarama are regarded as Hari and Hara. Here Balarama is regarded as Lord Siva. Siva is helping Vishnu in every incarnation like Rama-Laxman in Treta Yuga. In Dvapara Yuga as Krishna-Balarama and in Kali Yuga they are Jagannath and Balabhadra.
Some say that Krishna is Vishnu, Balarama is Shiva and Subhadra is Devi, thus the three siblings represent the three main schools of Hindu theism: Vaishnava, Shaiva and Shakta.