Are these your relationships?
Artworks::__pkArtworkID = Join::_fkArtworkID
Contacts::__pkContactID = Join::_fkContactID
Exhibitions::__pkExhibitionID = Join:_fkExhibitionID
(Substitute your field names for mine)
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
and sold by a contact (a gallery) to another contact (a collector)
Does that mean that there are at most, two contacts linked to any one artwork for given exhibition?
And when you set up a portal, is it a portal to the Join table?
and i cannot figure out how to add a second row to this portal.
If you are putting a portal to the join table on your Artworks layout, enable the "allow creation of records via this relationship" option for the Join table in your Artworks to Join relationship. (Double click the relationship line). If you place this portal on the exhibitions layout, enable "allow creation.." for Join in the exhibitions to Join relationship.
.....a single Exhibition in a portal in the Artworks layout, but i cannot figure out how to display related records automatically (e.g. select an exhibition title from a drop-down menu and then have the exhibition date and venue display automatically in that portal row
Make this a portal to the Join table on your Artworks layout. You can then add any fields from Exhibition or Contacts that you want directly to the portal row. When you select an exhibition in a drop down on the __fkExhibitionID field, those fields from Exhibition will automatically display data from the selected Exhibition table.
But given the "star join" between three different tables (if that's what you have here), the results that you see in your portal may not be what you expected. If artwork A is Exhibited at Exhibition 1 at Gallery X and Sold to Customer Y, you'd need two records in your portal, each linked to the same artwork and exhibition, but linked to two different contacts. That works, but may not be the result you wanted.