Use a tool like WinRAR or 7-Zip to open the archive. You usually only need to right-click the first file ( part1 ) to start the extraction process for the whole set.
If you are looking for more context on this specific archive or need help with it, here are a few common things to keep in mind: Golden_Cello_Decade.part3.rar
If you are mentioning this because you are having trouble with it, check to see if there is a .sfv or .md5 file included in the post to verify that the download wasn't corrupted. Use a tool like WinRAR or 7-Zip to open the archive
Since this is part3.rar , you will typically need all other parts (Part 1, Part 2, etc.) in the same folder to successfully extract the full contents. Golden_Cello_Decade.part3.rar