I have done it in the past. We have two phone lines in the house, and each one has had a separate sign up bonus, even LD company. Just make sure that when you sign up initially, and they ask if you have any other lines in the house (that you would like to sign up), say "No." By law, you can have a different LD carrier on each phone line, if you wish, even though they are all part of one account with your phone co. Thus, I think you can have three separate AT&T accounts for the lines. I am not 100% sure of this, so please correct me if I am wrong, but this has been my experience. The drawback to this is that you end up paying fees separately for each line, which can be quite pricey in the end.