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Om Shanti Shanti Shanti Why do mantras usually end with Shantih Shantih Shantih (3 times)?

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Om Shanti Shanti Shanti

At the end of most of Mantras from Vedas & Upanishads, we find Om Shanti Shanti Shanti. As in the end of this Mantra,

ॐ असतो मा सद्गमय ।
तमसो मा ज्योतिर्गमय ।
मृत्योर्मा अमृतं गमय ।
ॐ शान्तिः शान्तिः शान्तिः ॥
Om Asato Maa Sad-Gamaya |
Tamaso Maa Jyotir-Gamaya |
Mrtyor-Maa Amrtam Gamaya |
Om Shaantih Shaantih Shaantih ||

Meaning:
1: Om, Lead us from Unreality (of Transitory Existence) to the Reality (of the Eternal Self),
2: Lead us from the Darkness (of Ignorance) to the Light (of Spiritual Knowledge),
3: Lead us from the Fear of Death to the Knowledge of Immortality.
4: Om Peace, Peace, Peace.

As I want to know,

  • What is the purpose of adding om shanti shanti shanti at the end of most mantras?
  • What is the meaning of om shanti shanti shanti ?
  • Why does it written Only for three times? Why not more or less than three times?