How to tell if I have t1 diabetes, lupus and m.S at age 17?

Not likely. Why worry about 3 disorders, as you likely possess one unifying diagnosis to explain your problems. A good place to start is with an endocrinologist to deal with diabetes and the complications secondary to this. Focus on these issues, and later, if things seem peculiar look into co-morbities. Lastly, if you possess diabetes and lupus, doubt you have ms in addition.