Formula Excel Help

Hello Community,

i need help to create a Formula - i would be very happy if someone can help me here.

I have more then 400 lines and i need to change all of them as below.

is the standard but i need this number as below:

Actuall i need to change every line by mysef to mention the  "-" after the first three numbers. 

Is there a possibility to have a Formula here that every number will be shown as xxx-xxxxxxxxx ?

Thank you all for your help!

Jenny