I am a self taught musician (guitar/keyboards) with a limited knowledge of written music theory. For example if you asked me to play an augmented fourth I wouldn't know what you meant but if I observe you play it I would understand exactly what you were talking about.
Regarding the infamous "Devil's Tritone" that was banned by the church in the middle ages. To my mind tritone implies three notes, yet I only know of two which are used extensively in Heavy Metal for the evil, doomlike quality it lends a composition (Enter Sandman by Metallica is an example: E/A#). Dependent on which key you are playing in - E/A# F/B F#/C G/C# and so on.
My question is all references and instances of this only seem to use two notes. What is the third note that composes the Devil's Tritone?