Not sure which software are you using. In any case, the water discharge at a bifurcation is not presribed, as it is the results of the hydrodynamic set of equations. Worded differently, given the upstream discharge and the water level at the downstream ends, the water discharge in each branch is uniquely given, as the water level in the bifurcation must be the same for the 3 branches. If you are thinking about sediment partitioning, what you have to specify is not the sediment transport but the nodal point relation (i.e., a relation between discharge and sediment transport for the branches).