This is according to a report from Recode who claims that Apple has approached the companies about the idea, where their Apple TV or iOS users could subscribe to all three of them at once. This is almost similar to models offered by traditional pay TV operators, but the difference is that this bundle will be sold as a standalone product, whereas TV operators typically require users to at least subscribe to basic level of TV channels first.
It is unclear if the bundle would be priced cheaper than if you were to subscribe to them individually, where HBO costs $15, Showtime at $11, and Starz at $9 a month, but presumably it would be otherwise what would be the point of the bundle, unless Apple has some other freebies or perks thrown in, right?
The report from Recode claims that Apple has yet to finalize a deal with any of these networks yet, so for all we know it may never materialize, but basically the idea is that this is something that Apple is exploring.