It's best to first watch the first two movies, then read the novel. Then finally watch part III.
The thing is, reading the novel first takes away the suprise effect of the movies, and may drastically reduce the overall impression, also because of the novel you will perceive the film more critically. The novel is great to read afterwards as it provides details that were left out of the films. It's the definitive background check.
On the other hand, the novel has some surprise effects which the film lacks, especially the deaths of ... and ...
