Ian, the gain of 3 is the textbook gain to satisfy Barkaussen criterion to sustain oscillations, however, many times you need a gain of higher than 3 to guarantee oscillations every time the circuit starts, you need to move the poles to the right so the oscillations start growing, then as oscillations grow, the diodes conduct and if you place a resistor in series with them this resistor will be in parallel with the feedback resistor as the diodes begin to conduct, thus restoring gain to 3, the diode/resistor combination works as an AGC to avoid saturation. I've built these circuits dozens of times and they sometimes start oscillating with a gain of exactly 3, many times they do not. Check out Sedra/Smith's book or similar, I can give you plenty book examples like the one I am refering to.