Two Answers

The puzzle of the three princesses has been up far too long for how simple it is, I suppose its time for the solution.

Its actually quite easy, first number the princesses 1, 2, and 3 and then ask princess number 1:

Is princess 2 older than princess 3?

Then, based on the answer you get

yes: marry princess 3
no: marry princess 2

Easy enough, if princess 1 was the truth teller, then you aim to marry the younger one. If princess 1 was the liar, you aim to marry the older one. If princess 1 was the middle sister, then it does not matter who you marry, as long as it is not her.

I suppose that it was a bit of a lie when I said this is not a meta-question, since asking about the "truthiness" of other sisters is something of a meta-question, and asking about their ages is equivalent to asking about how much they tell the truth in this problem. I don't know though, the answer seems elegant enough, and its hardly going to count as a trick or "communicating badly" to have an answer involve their ages when it is said so clearly in the problem.

Anyway, yea, thats the answer as I know it, if you have any other possible answers feel free to say them in the comments.