Curtis-wiki is useful, but not the final word. Hear, see and speak no evil is just a catch-all phrase meaning stay on the positive side of life, IMHO. If you entertain evil, indulge in evil, speak evil it pollutes your own soul, why gossip? You take on that evil, it 'soils' your own soul. It doesn't mean we don't see things for what they are, we just don't indulge in them in the same manner. MMY was saying the same thing, I hope this helps you understand MMY better.
Behold, I send you forth as sheep in the midst of wolves: be ye therefore wise as serpents, and harmless as doves. Matt 10:16 King James