What are three diseases that can be caused by unhealthy eating?

Three diseases which can be caused by unhealthy eating are Type II Diabetes, heart disease, and tooth decay.

Over-consumption of sugar can increase the risk of developing Type II diabetes, a condition where the body develops insulin resistance.

Over-consumption of fat can increase the risk of developing coronary heart disease, a condition in which the coronary arteries which supply the heart muscle become blocked. 

Over-consumption of sugar and other acidic foods can lead to tooth decay, the bacterial breakdown of teeth.

None of these conditions are caused by diet alone. Genetic predisposition also plays a role. As do lifestyle habits. It is important to consume a variety of foods in moderation and to foster a healthy lifestyle by getting regular physical exercise to maintain a healthy weight.