Replace R1 and R2 with potmeters P1 and P2 and you should be able to change the frequency on pin 3. But be aware that the frequency depends on both potmeters.
With P2 you can change the LOW and HIGH timing : high = ln(2) * (P1 + P2) * C;
with P1 you can change the LOW timing : low = ln(2) * P2 * C;
wavelength = high + low = ln(2) + (P1 + 2*P2) *C; => so freq = 1/ wavelength = 1 / (ln(2) * (P1 + 2 * P2) * C);
The way to work is:
first make LOW timing correct with P1 , then adjust HIGH timing with P2.
Note that the HIGH time >= LOW time. ==> dutycycle is allways > 50%
Hopes this helps,
Rob