The human depiction of the Divine is symbolic and not literal. The various eyes, faces, arms, etc of Dieties have their own meanings. But since there are rare people who can explain them, we make the mistake of trying to understand the Absolute Power of the universe in human terms. I will give you an example - Vishuddh gyan dehaye, trivedi divya chakshuse. Shreya prapti nimittaye, namah som-ardh dharane. This dhyan of Lord Shiva makes it clear that the Absolute and Pure knowledge is depited in bodily form as Lord Shiva.
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This is purely the poets hypothesis of presenting Lord Shiva with Panchamukh to symbolise the PANCHAMAHABHUTA(five elements of energy i.e.- khyti, opp, tejo, marut, byoma).The concept of Lord Shiva's third eye emerged as Lord Shiva is presumed as the TERMINATOR of the Universe if he finds any proliferation of injustice.For this purpose itz redundant to represent this characteristics in all his five faces.