After releasing the first beta of iOS 9.2.1 to developers earlier today, Apple also seeded OS X El Capitan 10.11.3 beta 1 to members of the Apple Developer Program.
The first beta of what would become El Capitan's third major software update since its October 2015 debut carries a build number of 15D9c and is available as a standalone download through Apple's portal for developers or through the Mac App Store's Update tab on Macs enrolled in the beta program.