Has there been discussion if Apple's CarPlay will be available ?
It's not and it's highly unlikely it ever will be. I imagine Tesla sees it as a fig-leaf for legacy carmakers who can't make their own software.
Tesla will create their own software if they want something like this in their cars. Currently, does not exist. Will they make it? I presume if there is enough demand for it coming up with the extensive number of potential Model 3 owners. So if you want something like that, let your voice be heard.
I have a car with CarPlay, and it's a bit disappointing (currently). The big thing I'd like to see changed is the ability to view an alternative navigation system on the car's screen. I'd love to just run Waze instead of Apple Maps, but the best you can do is have Waze running on your phone but Apple Maps displayed on the car's screen, which is far from ideal. I'm fine with Tesla leaving it out until it's more useful.
Lots of talk about it in the past. Here's a in-depth analysis of CarPlay on the Model S. https://teslatap.com/articles/carplay-on-tesla/
Basically, most feel Tesla will not offer CarPlay (or Android Auto), as both would be a downgrade from Tesla's current system.