The HJ-Y3 tricopter frame comes with a mounting plate intended for the crappy TowerHobby MG-90 servo. In the particular frame set I purchased I also received the servo. The MG90 is not exact enough to be used as a helicopter or tricopter tail servo. It has also got other serious issues that makes it important to excahange it for something better.
I made an adapter that makes it possible to mount a Turnigy S306G high quality all metal servo as tail servo. No modification of the frame is necessary. The adapter slips over the servo and rests against the mounting posts.
Please note that the adapter has to be extremely tight to hold the servo in place. It is made a fraction to large on purpose so that it will not allow any wiggle room, regardless on which printer it is produced. You will need to use a file on both the outer and inner surfaces to make it fit perfectly.