Most trainers are setup as tri gear. I am one of the few who feels like a properly set up tail wheel can be better than a tri gear setup for training, mostly because after the first rough landing the nose wheel warps and the plane never tracks staight again. But it does take a little technique to tame a tail dragger in a cross wind, especially on pavement.

Hangar 9 markets a Mustang they claim is a trainer, and it is a tail dragger, but many (myself included) will debate whether or not it is actually an ideal primary trainer.