Hi Ochills,
If you carefully have read the description of Bollinger, you should remember that it is based on normal distribution. 2 or 3 is number of standard deviations from current price that is calculated based on lookback period. Thus "2" assumes 95% probability that price will remain between the bands if volatility will be the same or less than those that was put in calcuation of deviation.
3 assumes 99% probability. This is the difference. But signals by bollinger is quite different compares to oscillator predictor. Besides, using "3" you will have wider bands, while "2" will give you narrower bands.
But "2" or "3" do not change the core. You may use even "1' In this case probability will be 66.8% (or 68.6%) now I do not remember exactly.