And yes, the parol evidence rule may often be applied. The judge may say, "I see no reason to make any inquiry or do any other fact finding than what's in the "four corners of the contract"".
But let's take an extreme scenario.
Ten Catholic priests, ten Baptist ministers, ten Episcopalian ministers, ten Jewish rabbis, ten Muslim clerics, etc., etc. are all in the same room (perhaps a convention hall) and witness your being forced to sign a horrendously unfair contract at gunpoint.
Do you REALLY envision the judge, confronted with all these credible witnesses who are champing at the bit to testify, saying "the parol evidence rule always (100% of the time) applies and all I can do is look at the four corners of the contract"???!!!
Maybe, but there's a better than even chance that the "duress" part of Exception #3 may possibly perhaps maybe apply.