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thank you for your interesting post and questions.

The last one is what you want. The negation made by the not operator results in 1-x, where x is the membership function value that indicates how much of that proposition is passed onto the consequent. With this in mind, the results you get are different for each of the cases, as you mentioned. Specifically, look at how the propositions get translated:

1 - (a1 + b1 + c1) = if not(AAA is high or BBB is high or CCC is high)
(1-a1) * (1-b1) * (1-c1) = if AAA is not high and BBB is not high and CCC is not high
(a2+a3) * (b2+b3) * (c2+c3) = if (AAA is low or AAA is medium) and (BBB is low or BBB is medium) and (CCC is low or CCC is medium)

where + refers to the S-Norm used for disjunction, and * refers to the T-Norm used for conjunction.