Hi penelope,
I don`t know how deep your knowledge is regarding oscillators.
Nevertheless, the last page shows a classical WIEN oscillator topology - especially suited for 3 different frequencies.
Because the used amplitude stabilization principle (with filament lamps as PTC elements) requires to adapt the time constants there is a "low distortion mode".
I recommend, first to decide if you really need this specific oscillator with 3 fixed frequencies.
Otherwise, you should start from the basic WIEN topology.