As far as I know you can't have more than 2 master-detail relationships on 1 object, whenever you create more than one, you make what salesforce calls a 'junction object' which gives you the ability to create an many-to-many relationship.
Salesforce documented the way sharing settings work on junction objects quite well.
The first master-detail relationship you create on your junction
object becomes the primary relationship. This affects the following
for the junction object records:
- Look and feel: The junction object's
detail and edit pages use the color and any associated icon of the
primary master object.
- Record ownership: The junction object records
inherit the value of the Owner field from their associated primary
master record. Because objects on the detail side of a relationship do
not have a visible Owner field, this is only relevant if you later
delete both master-detail relationships on your junction object.
- Division: If your organization uses divisions to segment data, the
junction object records inherit their division from their associated
primary master record. Similar to the record ownership, this is only
relevant if you later delete both master-detail relationships.
I'd recommend reading the documentation for the rest of the specifics
In case you have the wrong master-detail relationship as the primary relationship, you can easily fix this. By temporary changing your current primary master-detail to a lookup, and changing it back.