I don't like to think in terms of percentage being dealt preflop as this is at times not realisitc.

While you can mathematically be dealt 6 different and unique AA. You find that in the actual game, there are only two AA hands at best, using all the aces . For example, when a person has the ace of hearts, another person can not have that ace of hearts.

You have 33 on a flop of 37A, and assuming you think the villian has either AA or AX (X=King, Queen, Jack). There is only one hand your afraid of and that's AA - the only combination of AA using what ever remaining aces in the deck you like.

But with a flop of 37A, he could have AX in three different ways, using any of the remaing three aces.

So, assuming you know your player. You just have to weigh it up and call. You lose to one hand AA, but beat the remaining three hands AX.

Although he's more likely to be dealt AK than AA, but according to your question, given the cirumstances that it is equally likely for the villian to hold AA as AK, equally likely as in 50% AK and 50% AA. Then it becomes a coin flip on your part.

For me to many people play AK stronly in this spot, that its hard to decide. Factoring in, of course, how s/he likes to play the nuts.