Thank you found the required link on LibreELEC forums.
I expected to find this information on the Five Ninjas forum to be honest since it's very specific to the Slice.
Posting it here if anyone else is trying to upgrade to CM3
Images (Preview, not considered as stable yet)
In there you will find LibreELEC releases for Slice Compute Module 1 and Compute Module 3.
Make sure you choose the proper file.
CM3 users should download LibreELEC-Slice3.arm-7.95.X files
CM1 users should download LibreELEC-Slice.arm-7.95.X files
Then flash your eMMC normally using the Normal Procedure
We warned that you might screw up your installation if you do it wrong so use it as your own risk.
If anyone has any questions on the matter I ll gladly help.
Gone tweaking Kodi 17 now!