"They are full."

Translation: Sunt plini.

[Tony602713]

One, Sunt plini, two, Ei sunt plini, maybe i dont understand

[-Afina-]

The "they" in the questions asks for plural, therefor it's "Ei sunt plini" but you can leave the "ei" because "plini" indicates that it is plural.
Singular, i.e. "I am full" would be "(Eu) sunt plin", again "plin" indicates the singular in this case and "It is full" would be "Este plin" if I'm not mistaken.