The type of relationship you are describing is called a Ternary relationship. In an ER diagram you would draw it as the rhombus sign connecting 3 entities.
I could explain it here but a better explanation with examples and also possible ways to consider converting a ternary relation to a set of binary relations is described here: http://www.databasedesign.co.uk/bookdatabasesafirstcourse/chap3/chap3.htm