There must be 17A flowing out of the other two wires
Electric current is a stream of moving electrons.
The number of electrons that leave a point has to be the same as the number of electrons that arrive there.
. If they arrive and don't leave, then there has to be a way to store them there, like a capacitor or a battery.
And if they're leaving but not arriving, then there has to be a tiny factory there, manufacturing electrons and shipping them out.
The whole idea is called "Conservation of electric charge". The idea is that charge can't be created or destroyed. If charge appears, it had to come from somewhere. And if you have some that you don't want, you have to send it somewhere, because it never just disappears.
There's actually a law in electronics that covers this nicely, called Kirchhoff's current law. It says that the sum of all currents entering a single point is zero. (current coming in is positive, current going out is negative). In other words, All current In and all current Out are equal.
