I had a similar problem and found this tutorial showing how to modify an existing part. I was surprised at how easy it was. In your case, just start with the three-digit part.
If you have the manufacturer's part number, see if you can find it on Mouser.com. You can get the footprint from then.
Click on "Build or Request". They will ask for an email address and put your request into the queue. Mouser is pretty agressive to making libraries for every part and customer requests go to the front of the line. I did one request and got the component library the next day. No charge and no nagging me to buy from Mouser.
You will need the Library Loader program to convert the file to Eagle format. I don't recall where in the process, but you are given the option to make the converter a plug-in for Eagle.
