Ok, there is a lot wrong with your post. But I am going to skip past all the incidental wrongness and get strait to the core of your failure of thought:
"Is The Order 1886 worth buying?" does not mean "Is The Order 1886 worth playing?" It means "Is The Order 1886 worth owning?"
What you are describing is, essentially, a 3 day rental that allows you to play and complete the game once (being generous about the length.) Now, there are very, very few games I would rent for $15-$20. In fact, any game I can think of that I would be willing to rent for that much is a game I would be willing to buy for full price.
Now, can a 6 hour game be worth $60? Stated differently, can entertainment be worth $10 an hour? Certainly. I have paid more for an hours entertainment before and walked away completely satisfied. But it takes a high level of quality for entertainment to be worth that much. And The Order just does not look that good.
So, to answer your question, is The Order 1886 worth $15-$20? No. Not within the parameters you have set. Frankly, I doubt it would be worth it even to own the game at that price. It just doesn't look that good. I would not pay $3-$4 an hour for the level of entertainment The Order can bring me.