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This Python 3 program runs in 41ms.

Solution:The remaining letters in each row, from top to bottom, are: G, B, A, E, F, C, H, I, D.There is only one solution for the grid, shown below:

For 9 letters each occuring 3 times, it is feasible to do a brute force search of the 3**9 = 19683 possible options of distinct letters, but this solution wouldn’t be scalable.