Current through a short circuit in the Wheatstone Bridge configuration
First : Of course current flows through R3 and/or R5. They - and nothing else - connect to one end of a current source. As it is sourcing current, it must flow somewhere, and that means either R3 or R5 or both.

Second : Re-draw the circuit with nodes 1 and 2 shorted. Can you see that R1 and R4 are now in parallel? The same is true of R3 and R5. That lets you draw a much simpler circuit. You can now work out the voltage between node 3 and node 1/2 - ditto node 4.

At which point you know the voltage across every resistor, therefore the current through each. (Check your math : the R1 and R4 currents must sum to 20mA, ditto R3 and R5).

And having computed these, you can probably see how to calculate the answer to your question.