  Starting from 2003, a large number of the so-called exotic hadrons, such as
$X(3872)$ and $D_{s0}^*(2317)$, were discovered experimentally. Since then,
understanding the nature of these states has been a central issue both
theoretically and experimentally. As many of these states are located close to
two hadron thresholds, they are believed to be molecular states or at least
contain large molecular components. We argue that if they are indeed molecular
states, in the way that the deuteron is a bound state of proton and neutron,
then molecular states of three or more hadrons are likely, in the sense that
atomic nuclei are bound states of nucleons. Following this conjecture, we study
the likely existence of $DDK$, $D\bar{D}K$, and $D\bar{D}^{*}K$ molecular
states. We show that within the theoretical uncertainties of the two-body
interactions deduced, they most likely exist. Furthermore, we predict their
strong decays to help guide future experimental searches. In addition, we show
that the same approach can indeed reproduce some of the known three-body
systems from the two-body inputs, such as the deuteron-triton and the
$\Lambda(1405)$-$\bar{K}NN$ systems.
