Re: My rear view mirror just fell off! Help!
From what I've experienced with these sorts of clip mechanisms, you need to bring two of the prongs up to two faces of the bolt... ie: the mirror tilted at probably 30 degrees to it's final alignment.
Once you have the two points of contact made, you apply a "shear" force to compress the two prongs, as you roll the mirror assembly down to connect the third prong with the bolt.
I've rewritten this twice and it's still not real easy to digest, but it's the best I can do mate.
2014 Overland CRD
Perth, Western Australia