Final answer:
The mystical word used in mantras that contains 3 sounds but is represented by 2 letters in English is Om.
Step-by-step explanation:
In the English language, the word 'Om' is used to represent a mystical sound or vibration often associated with Hinduism and Buddhism. It is believed to be the sound of the universe, encompassing all creation.
In Sanskrit, the word 'Om' is written as 'ॐ' and consists of three sounds - 'A', 'U', and 'M'. When transliterated into English, these three sounds are represented by the two letters 'O' and 'M'.