Hello,
1 - Sorry, the "Vendor group" is implemented for the Shipping methods but is not available for Payment methods since a payment method is more "private" since it allows to send money to a specific account (and specific credentials).
2 - Vendor groups are Joomla groups.
You can assign a vendor to a group, in the tab with the ACL.
It was first used to simplify the ACL configuration, but it can also be used for shipping method assignation (but sorry, I was mistaken with the payment methods).
3 - Payment without a vendor assignation will be displayed for any vendor or only your store depending on the setting "Show only vendor payments".
In your scenario, "Payment method 1" and "Payment method 3" are not assigned to a vendor ; thus they will have the same behavior in HikaMarket : be displayed to your store (or to every vendor if you do not have the mentioned setting).
If you want to show "Payment method 3" for "Vendor B", you have to edit that payment method and modify the "Vendor Options > Vendor" setting.
Regards,