I have looked around for the notes or chords to play the typical three note (wah wah wah) fail sound effect. I believe it's typically played on a trombone which gives it that great slide effect. In trying to recreate this on a guitar it seems that any dissonent chord would work. For example, I took a D9 (xx0210) and moved it to the 4th fret and simply slid it down in half steps. It sounds close and can be done with nearly any open chord shape. The D chord variation just sounded best to me. Does anyone know if there's an "official" way to play this on the guitar?
When played on a trombone, it's just single pitches, as you can only play one pitch at a time on a trombone (excluding multiphonics). I gliss (slidey-effect) up to Bb, then A, then Ab, then G, with a molto ritardando from just before the Ab to the G, then give some slide vibrato on the G. It's pretty simple; all about feel.
On guitar, you can try to bend the notes a half step up… so fret an a, but bend up to the Bb, then fret an Ab but bend up to the A… I'd try that if you're using an electric, but (at least for me) it'll take some effort to smooth out the effect. If that doesn't work, see how a slide from the fret below sounds. I don't have my guitar with me, or I'd try it out. An effect pedal (don't ask me which, I play acoustic) may help to thicken up your tone to sound more like a trombone.
What you did may also sound alright, it just depends what sound better to your ear. I personally hear it as single pitches, not chords.
You're own description of the fail sound---"(wah wah wah)"---gives a big hint: use a wah pedal. I'd play around with the tremelo bar too, if you have one. I think, although I haven't actually tried it, that rather than falling off the note after the attack, I'd depress the bar before the attack and release it. But again, play around with it and see what works best.