Step 1: Reverse the second half of the alphabet.
The second half of the alphabet is:
\[
M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z
\]
Reversing this gives:
\[
Z, Y, X, W, V, U, T, S, R, Q, P, O, N, M
\]
Step 2: Construct the new series.
The modified series becomes:
\[
A, B, C, D, E, F, G, H, I, J, K, L, Z, Y, X, W, V, U, T, S, R, Q, P, O, N, M
\]
Step 3: Find the twelfth letter and the seventh letter to the right of it.
The twelfth letter from the left end is \( L \).
Counting seven letters to the right of \( L \), we get:
\[
Z, Y, X, W, V, U, T
\]
Thus, the seventh letter to the right of \( L \) is \( U \). Therefore, the correct answer is 3. U.