I have spent a lot of time trying to find uC/OS III in Atmel Studio. It may be that Atmel Studio only has support to make it easy to use the RTOS.
I found this brief statement but no other info. Seems the RTOS will be in Atmel Gallery.
Micrium uC/OS III (coming soon)
•Popular, commercial RTOS
•Example project, including uC/OS III kernel source code
•Support for SAM3 and SAM4 architecture
•Available through Atmel Gallery