Actually, in the first week of November, you will have about 3x the AP multiplier you have this week. With putting in not more than the every-day effort of All AP world quests, couple of invasion points, Emissary, Daily Heroic, and maybe 1-2 m+15 a day you can easily farm around 300bn AP in 1 ID. Every ID afterwards you get 30% more, so 390bn in 2nd ID, 507bn in 3rd ID (15-21 Nov).
That's 1,197bn (or 1.2 trillion) AP in 3 weeks before the raid opens.
So: Yes, it is doable within 3 weeks. And that's from 0 to 75, not from 66 to 75. Of course, it will be around 3-4 hours per day played needed for it. But then again: Casuals wouldn't need the 75 anyways and somewhat hardcore Raiders put these hours in anyways. So all is well