Any one explain the following guru mantra, as to why a common human guru is being compared with lords(Brahma, Vishnu, and Shiva)?
Gurur devo maheshwaraha
Gurur sakshaat para brahma
Tasmai Shri Guruve Namaha
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The sloka mentioned by the OP is a popular Guru Stotra sloka from Viswasara Tantram.
Brahma, Vishnu, Shiva are considered to represent 3 Gunas, ie., Rajas, Sattva and Tamas. And, Brahman is termed as trigunAtIta,ie., beyond all Gunas.
Guru is the one, who attained Brahman, and thus BRAHMAN itself. So he was eulogised as the equal to Brahma, Vishnu, Shiva.
There is another sloka from Guru Gita (P.24 - sloka 25) with slight variation, albeit with the similar meaning.
gururbrahmA gururviShNurgururdevo maheshvaraH |
gurureva paraM brahma tasmai shrIgurave namaH ||
Instead of Gurur sakshaat in the sloka quoted by the OP, this sloka contains gurureva.
It tries to give an idea of highest form of Advaita, according to which each individual (he/she) is a BRAHMAN, though unaware of that fact. He/she realises that himself/herself is the BRAHMAN, through the grace of his/her Guru, who is also BRAHMAN.
Rig Veda and Satapata Brahmana says the same in a slightly different manner.