I prefer a highback as they position the shoulder belt correctly at the neck, otherwise the belt cuts across the neck which is painful, and then there is a tendency for the child to put the belt under the arm or behind them to be more comfortable.
Our highback also locks the belt off at the shoulder, so the belt can't be extended, whereas normally the belt would be extendable.
Personally, I am just more comfortable with the highback, and my dd is 6, I would definitely not be comfortable with a 3 year old in a backless booster.
Bev