|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 1.1 to 1.6 kHz than the curves for G#3.
For general comments about the chalumeau register, see E3.
Sound spectrum of a Bb clarinet played using fingering for A3.
For more explanation, see Introduction to clarinet acoustics
You can hear A3 played.