  We investigate the appearance of islands when a closed universe with gravity
is entangled with a non-gravitating quantum system. We use braneworlds in
three-dimensional multiboundary wormhole geometries as a model to explore what
happens when the non-gravitating system has several components. The braneworld
can be either completely contained in the entanglement wedge of one of the
non-gravitating systems or split between them. In the former case, entanglement
with the other system leads to a mixed state in the closed universe, unlike in
simpler setups with a single quantum system, where the closed universe was
necessarily in a pure state. We show that the entropy of this mixed state is
bounded by half of the coarse-grained entropy of the effective theory on the
braneworld.
