Along with the announcement about Windows Phone GDR3, Microsoft announced the new Windows Phone Preview program for Developers. The program will be officially launched later today and it will give app developers early access to OS updates so that they can verify that their apps work as expected on the new code. If you have a developer unlocked device, you can download the latest updates.
To participate and download Windows Phone 8 Update 3, you need to meet one of three conditions: your phone is “developer-unlocked,” you’re a registered Windows Phone Store developer, or you’re a registered Windows Phone App Studio developer.
More details about this can be found here.
Getting Update 3 on your developer phone
To update your phone, you’ll need to visit the Windows Phone Preview for Developers page. This page, which will go live later today, goes through everything you need to know about the preview program. Once you accept the terms and conditions, you can download the Windows Phone Preview for Developers app, which installs from the Windows Phone Store directly onto your phone and allows you to opt your phone in to receive updates. Then you can head over to your phone’s Phone update setting app and get your update on.
To participate, you need to meet one of the three conditions: your phone is ‘developer-unlocked,’ you are a registered Windows Phone Store developer, OR you are a registered Windows Phone App Studio developer.
Source: Windows Team Blog