Distributive laws are a standard way of combining two monads, providing a
compositional approach for reasoning about computational effects in semantics.
Situations where no such law exists can sometimes be handled by weakening the
notion of distributive law, still recovering a composite monad. A celebrated
result from Eugenia Cheng shows that combining $n$ monads is possible by
iterating more distributive laws, provided they satisfy a coherence condition
called the Yang-Baxter equation. Moreover, the order of composition does not
matter, leading to a form of associativity. The main contribution of this paper
is to generalise the associativity of iterated composition to weak distributive
laws in the case of $n = 3$ monads. To this end, we use string-diagrammatic
notation, which significantly helps make increasingly complex proofs more
readable. We also provide examples of new weak distributive laws arising from
iteration.