It's late here and my thoughts are a bit jumbled, so bear with me....
The 3 year thing is just a guidline - a sort of "educated guesstimation", if you will. How much B12 a person really
has stored up will depend on a million and one factors, none of which I can cover right now.
For this reason, some will say "you have about 3 years worth of B12 stored up" while other say that the "3 years rule" is dangerously misleading hogwash and you should start supplementing as soon as you stop eating meat/eggs.
Dr. Bernard is right, though - a good multi often will have enough B12 in it so if one is taking a good multi, they shouldn't really worry about getting enough B12. However, there are factors that will affect B12 absorption so the "shouldn't worry" part is a blanket generalization. As you know, it's impossible for doctors speaking about nutritional issues to address each and every circumstance that exists in each and every person, so they rely on broad generalizations to explain things on a basic level while leaving the specifics up to the individual's own physician.
The best thing to do IMO is supplement B12, whether in a multi or by itself. Some do this daily while some feel that weekly/bi-weekly dosing of B12 is sufficient. I know for me, I feel the best when I supplement daily/every other day so that's the schedule I stick to (I don't take a multi ATM so I'm super cautious about keeping my B12 levels nice and high).
There are a lot of very educated science heads on this board that are very knowledgeable about B12 and I'm sure one of them will come along and give much better info than I just did, but I hope I cleared a few things up for you (maybe?).