Need help planning relationships

I have a project tracking contact (phone, email, etc) activity related to sales. I have 3 CPTs: Activities, Contacts, and Companies.

When I create a new Activity, it will be associated with Contacts, a Company, or both. An Activity can be associated with none, one, or multiple Contacts. An activity can be associated with none or one Company.

Companies have contacts, but not every contact has a company.

Do I want to create relationships between Activity and Contacts and Activity and Companies and a sibling relationship between Contacts and Companies?

What's going to be the best way to set up these relationships?

Thanks!