'Interference (Book One)', written by Lawerence Miles, features the Third Doctor, Eighth Doctor, Sarah Jane Smith, Sam, and Fitz Kreiner.
Back Cover Blurb
Five years ago, Sam Jones was just a schoolgirl from Shoreditch. Of course, that was before she met up with the Doctor and discovered that her entire life had been stage managed by a time-travelling voodoo cult. Funny how things turn out, isn't it?
Now Sam's back in her own time, fighting the good fight in a world of political treachery, international subterfuge and good old fashioned depravity. But she's about to learn the first great truth of the univers, that however corrupt and amoral your own race may be, there's always someone in the galaxy who can make you look like a beginner.
Ms Jones has just become a minor player in a million year old pwer struggle... and as it happens, so has the Doctor.
Both of him, actually.