I got the Detailed instruction for installing this ROM here : http://www.droidphoria.com/2016/05/xperia-z3-marshmallow-update.html

Xperia Z3_D6646_23.5.A.0.570_US_Generic 6.0.1
Sony Xperia Z3 (D6646) Android 6.0.1 firmware - Generic US
Sony Xperia Z3 (D6646) Android 6.0.1 firmware - Generic US