I believe that Cinemax for a month for free is the current promotion for existing customers. However, I could be wrong. I know that the 3 month wording on the premiums page makes it seem like it applies to all with AutoPay and Paperless Billing.
free hbo/showtime hasn't been around in the year that I've been a customer. Only thing I've ever received was HD Platinum for 3 months, Starz for 3 months, Encore for 3 months, and cinemax for 1 month. Now I've got Starz + Platinum free for 12 months.. Wish there were a freebie for hbo/showtime!
I believe such promos generally happen late summer/early fall. I was able to get it last year so I could watch Boardwalk Empire (HBO) and Dexter (Show). It could have been because HBO wanted to hype Boardwalk.