I have some professors that are willing to give me letters of recommendations for medical school.

I have some professors that are willing to give me letters of recommendations for medical school.  The problem is that I have not taken the MCAT and I have to wait until next summer to take it which means I will not be eligible for med school until fall 2019.  One of the professors said that he will send the letter of recommendation once the clearing house sends a request or once I give him the address of where I would like it sent.  The problem is that I want to be able to send it to multiple places so I can’t give an address.  I need to collect the letters of recommendation now so that I have them for in the future.  What is a clearing house and is it too early for me to start an account with the clearing house so that they send requests for the letters of recommendation?