A child object with multiple parent objects is not available for selection as Target Object in 'Create a Record' quick actions, including situations where standard objects behave like master-detail relationships (such as Account and Opportunity). In any of these situations, Salesforce does not allow to create the actions to related child entity when we have multiple relations of different types to the child entity. For Example:
This is a Working as Designed behaviour. For Master-Detail relationships, a detail object record requires a master record when child record creation takes place, where as in Lookup relationship a detail object record can take place with/without requiring a master record.
However, as a workaround, in order to create an action with C as a target object on B, you can use any of the below options but please review Considerations for Object Relationships before changing the relationships:
The workaround suggested in Cannot choose a specific custom object as the target for 'Create a Record' Global Action (to create an action for detail object prior to establishing the master-detail relationship field) is also applicable, however this is not advisable.
001927677

We use three kinds of cookies on our websites: required, functional, and advertising. You can choose whether functional and advertising cookies apply. Click on the different cookie categories to find out more about each category and to change the default settings.
Privacy Statement
Required cookies are necessary for basic website functionality. Some examples include: session cookies needed to transmit the website, authentication cookies, and security cookies.
Functional cookies enhance functions, performance, and services on the website. Some examples include: cookies used to analyze site traffic, cookies used for market research, and cookies used to display advertising that is not directed to a particular individual.
Advertising cookies track activity across websites in order to understand a viewer’s interests, and direct them specific marketing. Some examples include: cookies used for remarketing, or interest-based advertising.