Playing 3 notes at once?

[Nabby]

For some reason I find it impossible to play 3 notes at once if one(or more of the notes) are harmonics, esp. in victor wooten's rendition of Amazing Grace (although those tabs are probably wrong), and Continuum. Any suggestions? as it always ends up in me just making a muted percussive sound, unless the treble is ridiculously high, and even then you can only hear a very, very quiet harmonic (or is this normal?).