Hello,
I'm sorry but the display of the buttons is related to the template rules because you're using the legacy checkout ; there is no specific modification between HikaShop 2.6.4 and HikaShop 3.x for the display.
Afterwards, you might want to deactivate the "bootstrap design" if your template is not compatible with Bootstrap 2 ; but once again that's not something new of HikaShop 3.
I can see that the website is using HikaShop Business but your license is attached to another website.
Please understand that our support is for one single website.
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.