Should I contact the universites that have not given me a decision yet after I have taken a offer? I am a student who just finished applying for a Ph.D. in Physics this year. I have not only received and but also accepted the offer from my dream school. However, there are three other universities that have not given me a decision yet. Should I contact them to let them know that I will turn down their offers anyway?

If the answer is yes, how should I word it so it doesn't sound weird?
It would be polite and helpful for you to let them know as it reduces their work. But a simple mail/email is all that is required.

"Thank you for your consideration, but I've accepted another offer." 

No more is really needed.