Google has officially launched Android P's third developer preview and the images for the preview are already going live, meaning if you have the correct device, a device that's compatible, then you can install the preview onto your device by flashing the image to it. Since these are just the images and not the over-the-air update software uses shouldn't expect to have the software pushed out to them at this time, though Google will eventually push the software to those who are opted into the Dev Preview beta, so it will just be a matter of waiting for those who aren't comfortable with flashing the images to their device.
In regards to the changes there are only seemingly a few that Google mentions in the changelog, which includes and updated set of permissions that the system needs to access things like call logs and the phone numbers listed in them as well as in the contacts list. This latest version of the Android P Dev Preview also includes changes to the security, albeit a small change. Based on the notes from Google there isn't a whole lot that has been added here, at least on the face of things, as there may be more changes that will pop up over the course of days and weeks following the installation of the new preview by users. This has been the case every other time with Google has released a dev preview beta build, as not all changes to the software, meaning new features too, have been listed in the release notes and changelogs. It does however add in 157 new emoji, including people with red hair, a bagel with cream cheese, a lobster, and even a llama.
This being a beta it's also important to remember that this is not completely stable software. There is actually a fair amount of issues that are known about at this point with this build, including hiccups with devices running the software communicating with NFC readers, which means issues with Google Pay. There are other connectivity issues too such as problems with playback over Bluetooth and not being able to adjust the volume for a device paired through Bluetooth to another device. This doesn't mean all users will have these issues, but it's something to keep in mind if you're planning on updating.