Why are the three buddhas in the Hall of Ursa Major exactly alike?

It's basically the same mold.

But their gestures are different and their instruments are different, from which we can tell that they represent Buddha and Bodhisattva.

According to the teachings of Buddhism, the Buddha is the same, there is no difference, there is no difference, and what his specific image looks like is revealed according to the needs of all beings. For example, the images of Buddha and Bodhisattva that you see on the mainland are perfect and festive appearances that China people prefer. When you go abroad, you will probably become a foreigner.

It doesn't matter what you look like. The important thing is that he is a Buddha and you are a Buddha.