Re Advanced Placement classes: I graduated high school in 1985 and took six A.P. classes over two years (three in Junior year and three in Senior). Lois could certainly have come into college with enough credits to let her graduate in three years.
All this is assuming that the writers are consistent across episodes. The simple answer is that the writers can't add and haven't thought through the entire timeline of Lois' college career. :p
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