Two days after seedingOS X 10.11.4 El Capitan beta 2andiOS 9.3 beta 2to members of theApple Developer Program, public beta testers who are signed on theApple Beta Software Programcan now freely deploy these new beta builds on their Mac, iPhone, iPad and iPod touch devices.
iOS 9.3 beta 2 is available as an over-the-air update inSettings → General → Software Updateon enrolled devices. OS X 10.11.4 beta 2 can be downloaded through the Mac App Store’s Purchased tab for those who are enrolled in Apple’s beta testing program.

As Jeff highlighted, iOS 9.3 includes several new features such as Night Shift, secure Notes, new 3D Touch actions and more.
New in 9.3 beta 2 is the addition of a Night Shift toggle in Control Center. You can simply toggle the mode on and off, or choose to have it stay on until the next day.
iOS 9.3 beta 2 also brings a redesigned Night Shift settings panel in the Settings app and a newAdd Playlist Songs to My Musictoggle in Music settings, while removing the uselessSet Wallpaperoption from Settings app’s Quick Action menu.
More information about iOS 9.3 can befound on Apple’s website.
OS X 10.11.4 beta 2 focuses on compatibility improvements in iBooks, Messages, Notes and Photos. After it releases in March, the fourth major update to El Capitan will enable viewing and sharing of Live Photos through iMessage in the Message app and support for protecting notes in the Notes app with a password.
iOS 9.3 will also allow iPhone, iPod touch and iPad owners to protect their notes with a password or Touch ID.