|Acoustics of the clarinet|
Notes are the written pitch.
Although it doesn't feel like it to your fingers, acoustically speaking, this fingering is actually a cross fingering: it has a closed tone hole below the first open one. This is responsible for the shape being more irregular from about 1.0 to 1.6 kHz than are the curves for F3 and G#3.
For general comments about the chalumeau register, see E3.
Sound spectrum of a Bb clarinet played using fingering for G3.
For more explanation, see Introduction to clarinet acoustics
You can hear G3 played.