unit is only three y/o. just stopped blowing cold.

We are all volunteers here rixter, so no need to ask your question twice; you will have to wait until someone knowledgeable comes on to answer your question.

edit:make that three times

You really should never have to add refrigerant to an a/c system, unless of course there is a leak, in which case, the leak must be located, fixed, and the system correctly recharged. I would suggest having the original installer back to check the system, it may not be a charge issue. Lots of options that can cause a system to not work properly. Good luck, and please let us know what is found.