In the mixed alphabet series A, B, N, C, D, O, E, F, P, ?, ?, ?, find the three letters that should replace the question marks.

Introduction / Context:
This series interleaves triples of letters. Each set of three letters appears to follow a similar pattern, and the task is to recognise how the first two letters in each triple relate to the third one. Once that pattern is clear, we can generate the next triple and thus fill in the missing letters.

Given Data / Assumptions:

• Series: A, B, N, C, D, O, E, F, P, ?, ?, ?
• We can group these as triples: (A, B, N), (C, D, O), (E, F, P), (? , ?, ?).
• The same structural rule applies to each triple.

Concept / Approach:
Within each triple, the first two letters are consecutive in the alphabet, while the third letter is some fixed number of positions ahead of the second letter. Observing how far the third letter is from the second letter in each triple allows us to project the rule onto the next triple and determine the missing letters.

Step-by-Step Solution:
Step 1: Look at the first triple: A, B, N. Positions are A=1, B=2, N=14. The jump from B to N is 14 - 2 = 12.Step 2: Second triple: C, D, O has positions C=3, D=4, O=15. The jump from D to O is 11.Step 3: Third triple: E, F, P has positions E=5, F=6, P=16. The jump from F to P is 10.Step 4: The jumps from the second to the third letters are 12, 11 and 10, decreasing by 1 each time.Step 5: The next triple should start with G and H as consecutive letters, and the third letter should be 9 positions ahead of H.Step 6: H is letter 8, so 8 + 9 = 17, which corresponds to Q. Therefore the last triple is G, H, Q.

Verification / Alternative check:
Collecting the pattern, we have triples (A, B, N), (C, D, O), (E, F, P), (G, H, Q). The offsets from the second to the third letters are successively 12, 11, 10 and 9. This clean decreasing sequence confirms that the structure holds across all four triples, so G, H, Q is the only correct completion.

Why Other Options Are Wrong:
Triples such as J, K, L or G, H, I ignore the decreasing offset pattern and instead try to continue with simple consecutive letters. Sets such as G, H, J do not respect the specific 9 letter jump from H, and therefore break the carefully observed pattern across the triples.

Common Pitfalls:
A common mistake is to focus only on the consecutiveness of the first two letters and guess any letter after them, overlooking the deliberate pattern in the third letter. Exam series questions frequently encode such gradually changing offsets, so checking differences in positions is essential.

Final Answer:
The three letters that correctly fill the blanks are G, H, Q.