I've done several updates from 2.11 to 3.1. Are you working with the live update or manualy?
If you wish I could try an update of your page in a mirror and write down the required steps. As I saw there seems to be no update instructions for english user. Here's the manual for germans https://contao.org/de/news/update-auf-contao-3.html
Some hints:
check that required extension are compatible
some not compatible extension may run after you do the autoload-creator you find in BE
don't rename tl_files to files till the end.
After the update use this script, to change file pathes in the DB https://gist.github.com/leofeyer/3304014
put this into localconfig.php
Code:
$GLOBALS['TL_CONFIG']['bypassCache'] = true;
$GLOBALS['TL_CONFIG']['coreOnlyMode'] = true;
$GLOBALS['TL_CONFIG']['displayErrors'] = true;
Put a .skip file in extension folders you like to deactivate
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