Why apple watch series 3 do not have Noise app???

You need at least an Watch Series 4.

The Noise app  on Apple Watch Series 4 and later measures the ambient sound levels in your environment using the microphone and duration of exposure. When Apple Watch detects that the decibel level has risen to a point where hearing could be affected, it can notify you with a tap on the wrist.

Note: The Noise app uses the microphone to sample and measure the sound levels in your environment. Your Apple Watch doesn’t record or save any sounds to measure these levels.

You're welcome.