Three princesses

[bentomat]

My solution:

Spoiler:

Sisters are, randomly, A, B, and C.
Ask A if sister B is older than C.
If the answer is yes, marry C. If the answer is no, marry B.
If A is the truthteller and B is the liar, the answer will be no. If A is the truthteller and C is the liar, the answer will be yes. In both instances, you marry the liar.
If A is the middle sister, it will not matter what she answers; you will marry one of the other sisters.
If A is the liar and B is the truthteller, the answer will be no. If A is the liar and C is the truthteller, the answer will be yes. In both instances you marry the truthteller.

[bananageek]

Edit: Sorry, completely misunderstood the question there!

[jestingrabbit]

You get to ask one question, to one princess, and get one reply, not one question to all the princesses, that gets three supplies.