Hello,
HikaShop 3.0 was a major release and we modified a lot of elements to propose big new features.
Due to the huge modifications we made, some "old" views of HikaShop 2.x might not work properly.
If the template you're using have some view override ; it is possible that you have some conflicts between the new HikaShop 3 system and some "old" views.
In such case, best would be to backup the HikaShop view overrides of your template and remove the ones related to the checkout and the cart.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.