It is known that a Feistel Network containing 3 rounds is relatively secure against Chosen Plaintext attacks. Now as a game I was given the task to find a possible attack against a Feistel Network with 3 rounds, but the first encryption with a key has no effect. (So the input in the second function is just $L \oplus R$). It should be possible to find out, with significant probability and only to Chosen Plaintexts, if the network produces the results of the chosen inputs or just a random sequence. I just can't wrap my head around it, can anyone help me there?
Thank you in advance!