As a system of 3 containers (X+Y+Z), the volume has not changed right?
But the better way to understand this is that the gases in the three containers are isolated from the surrounding. They do not exchange heat with the surrounding (so Q is zero), and the external pressure is also not causing the containers’ volumes to change (so W is zero). So the internal energy cannot change.
Also, before and after opening the tap, the total internal energy is still the total KE of all the gas molecules. This total KE may have been redistributed among the gas molecules, but the total remains unchanged.
xmPhysics 
hi may i know why internal energy still the same eventhough the volume has increased
As a system of 3 containers (X+Y+Z), the volume has not changed right?
But the better way to understand this is that the gases in the three containers are isolated from the surrounding. They do not exchange heat with the surrounding (so Q is zero), and the external pressure is also not causing the containers’ volumes to change (so W is zero). So the internal energy cannot change.
Also, before and after opening the tap, the total internal energy is still the total KE of all the gas molecules. This total KE may have been redistributed among the gas molecules, but the total remains unchanged.