I have spoken to Department of Finance officials, and I don't have direct knowledge of where the $3 billion came from. My guess is that it is essentially a holdover from the $3-billion contingency reserve that was used by previous governments, and it was just easily mapped into roughly $20 billion or $3 billion on federal revenues into the current forecast adjustment.
On your point about prudent forecasts in the budget, our reading is that the use of the forecast adjustment isn't really for prudence. At least, I couldn't find an explicit mention of that in the budget. Rather, it is about taking account of the downside risks. If you look back at the 2015 fall update and the February Canadian economic outlook, that is the way the language is presented, which is what you want to do as a forecaster. It is in that sense that we thought this forecast adjustment, specifically in the first year and the second year of the horizon, was excessive, and that this, at least on the surface, wasn't really about prudence. It was to balance the risks. You would really have to have a very negative scenario with a high probability of its being realized to justify a downward adjustment of $40 billion like that.