  In an in-situ prepared three-terminal Josephson junction based on the
topological insulator Bi$_4$Te$_3$ and the superconductor Nb the transport
properties are studied. The differential resistance maps as a function of two
bias currents reveal extended areas of Josephson supercurrent including
coupling effects between adjacent superconducting electrodes. The observed
dynamics for the coupling of the junctions is interpreted using a numerical
simulation of a similar geometry based on a resistively and capacitively
shunted Josephson junction model. The temperature dependency indicates that the
device behaves similar to prior experiments with single Josephson junctions
comprising topological insulators weak links. Irradiating radio frequencies to
the junction we find a spectrum of integer Shapiro steps and an additional
fractional step, which is interpreted by a skewed current-phase relationship.
In a perpendicular magnetic field we observe Fraunhofer-like interference
patterns of the switching currents.
