2- 3 cans of diet soft drinks like cola, iced tea or juice are probably not going to be harmful to your health or body. Most things like this in moderation are fine as long as you are not having any significant reactions to them. Some people do have negative health reactions that may be associated with the use of saccharin and aspartame products, like headaches and dizziness. If you are having any of these symptoms, you should definitely cut out diet drinks and foods and go to see your doctor.
There has been a great deal of research done by the medical community on the use of artificial sweeteners, but none proves directly any serious health risks or problems. This is not to say that it is completely safe because only time can tell this. Also, many people react differently to substances put in their bodies.
TeenHealthFX is glad that you are not solely relying on diet or sugar filled drinks as your only liquids. It is good that you are drinking a good deal of water. In fact, most dieticians recommend that people drink 6 - 8 8oz. glasses of water per day.