If I am taking lamictal would be harmful if I started taking diet pills such as 3 or fastin (phentermine)?

Not good idea. Diet pills such as these can make you feel more anxious and agitated, and in some cases could lower your seizure threshhold. Not advisable in any case. And if the Lamictal is for a mood disorder, diet pills could also lead to insomnia and further destabilization. Please talk with your own physician before using any over the counter medicine or supplement.