I'm trying to install to a Slice with brand new CM3 just fitted.
I managed to get it all working today Found a post about CM3 install on the Five ninjas forum that gave me so good starting points (thanks to tvoverbeek). Also some reading of the Raspberrypie.org documentation about flashing a CM3 (although their methodology uses an expensive CM interface board which I don't have). The key is to use rpiboot which enables the CM EEprom to be visible to the PC as a USB drive. I couldn't get my Win7 system to find the drivers so I switched over to my Ubuntu 14 box and followed the clone/make commands listed on the github site in this link: GitHub - raspberrypi/usbboot: Raspberry Pi USB booting code, moved from tools repository
Hope this helps anyone faced with similar upgrade.