Can a 3rd level ranger take boon companion feat?

[isildur]

Yes, because there is hunter's bond class feature on ranger's class feature list.
(although he can't get any benefit from boon companion)

No, because a 3rd level ranger does not yet have hunter's bond.

[Biffoniacus_Furiou]

No, you must already have a feat's prerequisite to be able to take that feat. He cannot take that feat until he's at least a 4th level Ranger, or has an animal companion from some other source.