Hi there,
I'm with the other mommies, let her nurse even if there is no milk. She will eventually stop nursing when she's ready. My little one nursed until she was 2 years 9 months! I always said I'd let her nurse until she was ready to stop, but was surprised it went on that long. I was a bit self conscious about being one of "those" mommies who was nursing a child who was that old, but it was just at night before going to bed. And what's the difference between "comfort nursing" and using a pacifier? There are plenty of 3-year olds who use pacifiers.
When I was about 5 months pregnant with our second child she stopped nursing one day and asked me for a glass of milk. I have no idea how long she was nursing without milk! And she asked to nurse several times after that, mostly when we were traveling or when she was really over tired. But she hasn't asked to nurse since she turned 3, even though she's been watching her baby brother nurse.
Great job breastfeeding for so long! And for listening to what your daughter needs and for being open minded enough to let her continue to nurse. Sounds like she will stop soon. :)
I forgot to mention, my daughter transitioned from nursing to holding a breast while falling asleep. Don't be surprised if your daughter asks to put her hand on your chest to fall asleep or other times for comfort. Seems like a natural next step in "letting go" of breastfeeding. :)