#1
The d string buzzes Only on that fret, any ideas why? I've had all the strings replaced before as well.
#2
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,
#4
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!
#6
A couple of light taps with a small hammer at that location should fix it.