I have done some maths, based on three runners that beat you that posted much more even splits, I have surmised that the timing mat could have been at something closer to mile 20.4 instead of 19.9 like it stated. In this scenario, you hit 13.1mi at 7.04min/mile pace. 20.4 at 6.96min/mile pace. Assuming you were able to maintain your pace from 13.1-20.4mi for the last 5.8mi, and assuming you actually did go off course, the best time I see out of that would be 3:01:40. In order to go sub-3 you would have had to close those last 5.8mi 17 sec/mi faster than you ran 13.1-20.4mi. I don't see it.
So, at the most optimistic I can possibly be, maybe you were in PR contention and missed a turn. But you still are not a sub-3 marathoner. Read the book, follow the book, report back next year when you run sub-3 by doing it the right way.
I can show my work if necessary.