The d string buzzes Only on that fret, any ideas why? I've had all the strings replaced before as well.
It sounds as if the third fret is a little too high and the D string is touching it when played - you can lightly rub down the area where the string crosses the fret with a piece of fine emery paper.
Last edited by Garthman at Jan 22, 2017,
Based on the quality of the guitar (and the setup, which it sounds like you should get checked out anyways), you may have issues using lower tension nylon strings.

I think that the setup, to include fret height as mentioned by Garthman, is most likely the culprit, as a well setup classical guitar should handle low and hard tension strings without a problem, but I recommend a second look before you begin altering the frets if you don't really know what you're doing.
My God, it's full of stars!
A couple of light taps with a small hammer at that location should fix it.