Out of all the three sentences, B is incorrect
A and C make sense however B doesn't
The sentence B (to me) tells that the beans get softer unless they are cooked.
Therefore, I would replace because with so, making the sentence more clear.
Often, they cook a large pot of beans early in the morning and let them cook so the beans get softer and tender. (I kinda want to put at the end of the sentence: "to eat" so the sentence explains why the beans are cooked)