Here's how you can find all three bell locations in "Black Myth: Wukong" to unlock a secret area and boss fight:
First Bell Location

First Bell Location in Black Myth: Wukong, image via Saber Interactive
This bell is located in the Forest of Wolves, near the "Outside the Forest" shrine. After defeating the boss Guangzhi in his arena, you'll find the bell at the back of the arena under an arch. Guangzhi's arena is accessible by following the path past the Wandering Wight, crossing a bridge, and heading towards a gate with flame-lit lanterns.
Second Bell Location

Second Bell Location in Black Myth: Wukong, image via Saber Interactive
Found after defeating Guangmou, which is near the "Snake Trail" shrine in the Bamboo Grove region. After the boss fight, instead of moving forward with the story, head to the left through the water and up some stairs where you'll find the bell.
Third Bell Location

Third Bell Location in Black Myth: Wukong, image via Saber Interactive
This bell is situated near the "Marsh of White Mist" shrine. After defeating the Whiteclad Noble, proceed through a path leading through water, keeping to the left. You'll encounter this bell after dealing with some wolf yaoguai.
What Happens After Ringing All Bells

What Happens After Ringing All Bells in Black Myth: Wukong, image via Saber Interactive
Once all three bells are rung, you'll be transported to the Ancient Guanyin Temple, a secret area where you'll face the Elder Jinchi boss. After defeating him, you'll obtain the Fireproof Mantle, which is your first Vessel, offering both active and passive benefits.
Final Thoughts
This quest not only provides you with a unique challenge but also rewards you with significant items that can aid in your journey through "Black Myth: Wukong". Remember, engaging with these bells and bosses might require some backtracking or exploration beyond the main story path, so keep that in mind as you navigate through the game's world.