The third sentence is correct. The first two are not. "To add up" refers to a number of smaller actions resulting in a larger end result. In the case of the third sentence, reading one book a month results in having read 12 books in a year. Most people would consider that to be a fairly large number of books to read. In the first sentence, the meaning of the sentence is that the internet makes a positive contribution to our lives. The sentence would be correct as: "The internet has really added to our lives." In the second sentence, the meaning is that being married without having love between the parents does not help a child's development. I would write the sentence as: "It does not help the child to remain in a marriage without love." I hope this helps.

Yes, they're all correct