You cant get three if it is in the BM deck. You can only get one.

Sure you can. As your deck bloats with cash, the risk of terminal collision is low, and you want to add some wharves to have a chance of drawing more than one per shuffle. Terminal collision is still possible, but if you have 25 cards worth of money and vp, then the chance of 2 wharves showing up in one 5-card hand are pretty low.

Let's use some conditional probability. I have a total of, say, 25 cards, including 2 wharves, with the rest being money/VP. I just reshuffled my deck and I didn't play a wharf last hand. The first time a single wharf shows up in my hand, it will appear with 4 other random cards. When I play it, I will draw 2 more, so I will see a total of 6 cards in addition to the wharf I just played. The chance that one of those 6 cards is the other wharf is 6/24, which is only 25%.

If you bought only one wharf, you've got an extra silver (or possibly a gold) in your deck, but I have a 75% chance to draw an extra 4 cards spread over 2 hands from my second wharf on every shuffle. That's a huge boost. In addition, since wharf is a duration, I can virtually ensure that I avoid terminal collision when I play a wharf just before a shuffle. The unplayed wharf is shuffled into my deck, while the played one stays out for another turn.

Bottom line: 2x wharf BM beats 1x wharf BM every time. Try it out for yourself.