AUM represents the Avasta Traya, the 3 relative states of Consciousness. Swana, Jagrat and Sushupti.
Aum is Braham, averred the Rishies. Om iti Brahma, OM iti Idam Sarvam !
While the Mantra of Vedanta is AUM, Tantra has a different Mantra. Hreem !
Hrillekha Sarva Mantranaam Nayika Parama Smrita
Hrillekha Darpane Nityam Aham Tu Pratibimbita !
Hreem became the Queen of all Mantras,
I am She who is Reflecting in the MIrror of Hreem
Since She is a Trinitarian Unity, each Aspect has been assigned different Mantras !
CREATRIX – AIM
PRESERVATRIX SREEM
EXECUTRIX HREEM
In other words, Aim, Sreem & Hreem represent the Trinitarian Unity and is called Tridaya.
I am the Creatrix, Preservatrix & Executrix,
I am the Rajas predominant, the Satwa predominanti & the Tamas predominant !
I was born in the Heavens,
And got weapons from the Great Three !
From an Esoteric perspective, Tra means Trayate and Mantra is that Saves the Chanter, Mananath Thrayate iti Mantra.
It protects you from your greatest Enemy, Mind.
Tra or Trayate is the base of many words,
Tantra
Mantra
Yantra
Putra
Sastra
All these will protect and save thee !