Hi,
Please understand that the issue doesn't come from the update itself even if it was the trigger.
The issue comes from the way the module was customized. Unfortunately, we can't do anything to avoid that.
With minimum customization made by the template on our demo website, we've been able to update it from HikaShop 2.0.0 up to now without much things to change to have it working like before while still being able to use the new features.
Please also understand that HikaShop 3 is a really big update. The biggest we've ever made. And we've changed the whole cart and checkout system with it in order to improve their usability and their capabilities.