I have a recently purchased and barely used Metz 3802 adapter from B&H. Works fine, but I have now obtained a Contax TLA 360 flash for my Contax 645.
This adapter will allow your Metz 53 MZ3 or 54 MZ3 to work in TTL mode with the Contax 645 body.
$75 shipped and paypalled.
I have lots of feedback on fredmiranda.com, same username.Link to B&H