Good news

The adbd binary is the same of 14.1.0:

$ md5sum tolino-1*/ramdisk/sbin/adbd
1d23e203eba05102e6cb642a117b8d64  tolino-14.1.0/ramdisk/sbin/adbd
1d23e203eba05102e6cb642a117b8d64  tolino-15.3.0/ramdisk/sbin/adbd

The direct download link for the new Shine 3 firmware is https://tolinodownload-a.akamaihd.net/ereader/15.3.0/OS44/update.zip.

As a sidenote, you can find the latest Tolino updates on the Tolino website.

Everything documented is still relevant

• Tolino Shine 3: TWRP without modifications
• Tolino Shine 3: take a full backup
• Tolino Shine 3: patch adbd to run as root
• Tolino Shine 3: use adb as root
• Bonus: this busybox works on the Tolino Shine 3