Two days after seeding iOS 9 beta 3 and OS X 10.11.4 beta 3 to developers, Apple has made the updates available to public testers. Folks who are registered in the company's Beta Software Program can now install the new firmware on their various devices.
As usual, the new iOS 9.3 beta is available over-the-air via the Settings app to those already running 9.3, and the new El Capitan 10.11.4 beta can be found in the Updates tab of the Mac App Store. You just need to make sure your Mac or device is enrolled.