Hello,
HikaShop 3.x is still compatible with old plugins based on "JPlugin" (while the usage of hikashopPaymentPlugin is highly recommended since HikaShop 2.x).
Now I'm sorry but I can't tell you the reason why your plugin is not working.
If the code is not being called ; I don't think the problem would be related to HikaShop since all the system is still based on Joomla plugins and Joomla trigger system.
When we developed the new system ; we tested it with a lot of plugin, even some old plugins in order to be sure that they are still working.
github.com/HikaShop/sample-payment-plugin
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.