தேவ பாடையின் இக் கதை செய்தவர்
மூவர் ஆனவர் தம் உளும் முந்திய
நாவினார் உரையின்படி நான் தமிழ்ப்
பாவினால் இது உணர்த்திய பண்பு அரோ.
In God’s language this epic
has been written by three;
Of them the foremost is whom
I follow to sing this in Tamil
In Kamban’s time there were three versions of Ramayana. One written by Valmiki, second Yoga Vaishta as told to Rama by Sage Vasisht, third Adhyatma Ramayana as written by Sage Vyasa. Kamban says of the three he follows the first, Valmiki Ramayana as the base of his epic.
There are more than three hundred versions of Ramayana. You can read this wiki for details. Also, you can read this 48 page artilce by AK Ramanujan if you want to explore further the various versions of Ramayana.