Most examples on the internet and in my text books contain 1 load with 1 source. I have yet to come across an example with 3 loads. Is there a way of combining the loads?
You know the line voltage so you can calculate the phase voltage and work out what line current the star load is taking per phase. You know what the delta load is so you can calculate the line current for it.
Add the two together to get total line current of the first two loads then subtract that from the total supply line current to get the current taken by the unknown load. The current that remains and the phase voltage define the impedance of the unknown load.