Final answer:
An unlikely crossover event for three linked genes A, B, and C in order would be a simultaneous exchange between genes A and B and genes B and C due to the proximity of these genes making such an event rare.
Step-by-step explanation:
If three genes (A, B, and C) are linked in that order, a crossover event that would NOT be observed would be one that involves a simultaneous exchange between genes A and B and between genes B and C. This type of crossover event is extremely unlikely because a single crossover event very rarely disrupts the linkage of two adjacent genes, especially when they are very close together on a chromosome. The recombination frequency is higher for genes that are farther apart, and so the recombination between genes A and B is more likely than between genes B and C, because there is more physical space for crossover to occur between the further apart genes.