Alongside releasing the first Developer Preview of Android P, Google also provided developers with an estimate of when future DP builds of the OS will land and when they should expect the final public version to be releasing. Google intends to release five preview builds of Android P before releasing the final version to AOSP and OEMs in Q3, 2018.
Google intends to drop the second Developer Preview of Android P in May presumably after its opening Google I/O keynote. The second preview of Android P should feature more user-facing changes and should also mark the beginning of Android beta program for the OS. The third DP of Android P with the final APIs and official SDK is currently scheduled to be released in June, with the first release candidate of the OS for testing scheduled for release later in the month. The final DP release and the second release candidate will be released sometime in late August before the final release of the OS in Q3.
The first couple of Android P Developer Preview releases are aimed at developers so that they can ensure their existing apps are compatible with it and also take advantages of the new APIs offered by it. With DP3, Google will finalize the APIs in Android P and also allow developers to publish Android P compatible apps on the Play Store for devices running the OS.
Google had followed a similar timeframe and schedule with the release of Android Oreo last year so it makes sense for the company to stick to a similar schedule this year as well. Remember that Android P is still in early stages of development and a lot of aspects of the OS can change before its final release in Q3 this year.