About the dissonant sound, with the 3rd. Usually when the 3rd is played in one voice, while the 4th is in another voice range. I'll explain better, say you play the C maj7 chord, the E is found on the 2nd fret of the D string. F will probably be played on the 1st fret of the high E string, or anywhere else on the fretboard (same pitch). There is dissonance, but in music dissonance plays the equal counter-role of consonance. Music that's all consonant is like watching a movie with only one actor, BORING!!!
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.