Jemaine Clement is a New Zealand comedic actor/singer/songwriter best known as being one half of the Flight of the Conchords duo.
In a recent interview he was asked about his MIB3 role, and whether he wears a “cool alien costume. This is his response:
“Yeah, I have prosthetics. I look hideous”
From the pics above, and his comment, it would be reasonable to suggest that this still from the official trailer is also Jemaine/Boris:
And finally, here’s a photo of Jemaine and his motorcycle trainer, Sarah Lahalih: