Triple head I have, <- Duh.
Luminance swathed are two. <- Not sure, but he/it IS located in a light pillar.
People plantation is swimming, <- SeeD-school, students - but I can't begin to puzzle out where the swimming comes into play.
Opposition dictation; possessed mother. <- The struggle against Edea, of course. To be terse.

Trade my card for,
The opposite of heavyweight,
by ten time ten. <- Poetry for 'Trade the Cerberus card for loads of Lightweights? Whatever, I think you get Haste out of them, at the end of the day.

I can warn you, <- It was a long time ago, but I believe he/it warns you that your hindquarters will be soundly whipped if you don't back off.
Upcoming combat; always face-to-face. <- My memory fails me, but do you perhaps have to approach him/it from his front to successfully interact with him?

Location: Center of entrance, <- Yeah, location's the word.
Dust hell gladly received, <- Not sure, but I believe Cerberus is some kind of hell's watchdog.
Gale sorcery I resist. <- Don't hit him with wind-magic, for thou wilt be remorseful.