What is special about the N3 buffer for Qiagen minipreps?
The Qiagen manual for the Miniprep Kit states that buffer N3 contains guanidine hydrochloride and acetic acid. The MSDS for buffer N3 gives a bit more information:


Guanidinium hydrochloride 25-50%
Acetic acid 10-25%
pH 4.3


Buffer P3 uses 3M potassium acetate, the potassium is usually important in plasmid preparations as potassium SDS is used to precipitate the proteins in solution.

This information might be enough to create a buffer that should work with the silica-gel columns.

I also found a patent about a method for isolating nucleic acids that states that silica particles can bind nucleic acids in the presence of chaotropic salts, so the guianidinium hydrochloride is probably the important difference in the N3 buffer.

The patent for this method from Qiagen goes into even more detail, according to this the buffer composition  is:


N3: 4.2M guanidinium hydrochloride, 0.9M potassium acetate, pH 4.8