It would be extremely difficult to "guess" at any type of adult color at this age. Different locality animals undergo ontogenic color changes at different stages in their lives.
Biak type locality animals are extremely slow in undergoing the color change, some taking as long as 3 years.
It's always a good thing to know about lineage when it comes to purchasing a Green Tree from someone. Folks who breed GTP's will gladly show you the parents to the neonate you are buying, or should be happy too in MHO!!! Anyway...
Biak and Aru locality type animals are the two largest races of GTP found, and I include the Yapen form of these animals.
You say you gave "3" for it. Is that 300 or 3000? If you gave 300 for it my guess would be it was a locality type Biak due to coloration.
Hope this helps.
Andy