Check
www.datafilesrus.com to see if there is a file there - otherwise it means that there probably isn't a file for AB3 yet. All the files are written by volunteers so there is no guarantee that a file will be available.
If this is the case you have two options:
1) Write the file yourself. Download ABCreator and have a go at creating the file yourself. It can be a steep learning curve but it is very satisfying when you finally get it working!
2) Search these forums for the Conversion tool to convert AB 2 files to AB3 format. How well this works depends on the files in question but it should help you get hold of a workable file.
Personally I recommend you have a go at making your own file. It can be quite fun - if a little bit frustrating!
Hope this helps.