Hello Yi, thanks for reaching out.
The Namespace allocation is not for traffic to a single experiment, it's to allow more than 1 experiment to be in the Namespace so that the experiments can be mutually exclusive.
If you need to run 3 experiments that all need to be mutually exclusive, and you want your traffic distributed evenly among the 3 experiments, you would set up your namespace allocation to be [0 to 0.33, 0.34 to 0.66, and 0.67 to 1] which would roughly be in thirds.
Does that make sense? Let me know if you have more questions.
https://docs.growthbook.io/features/rules#namespaces