RoR Attacks on modified Feistel Network
If you have three rounds and the encryption in the first round has no effect, then the left side of the third round is

$$ L_3 = R_0~\oplus~F(L_0 \oplus R_0, K_2) $$

and when you exchange the left and right side of the initial block

$$ L_3' = L_0~\oplus~F(R_0 \oplus L_0, K_2) = L_0~\oplus~F(L_0 \oplus R_0, K_2) $$

So

$$ L_3~\oplus~L_3' = R_0~\oplus~L_0 $$

This property lets you distinguish the output of the Feistel network from random.