Use software like WinRAR, 7-Zip, or The Unarchiver.
Based on the file naming convention, is the third segment of a multi-part compressed archive, most likely containing digital content from the Japanese platform DLsite . Identification of Content H-RJ435815.part3.rar
Right-click the part1 file and select "Extract." The software will automatically pull data from part3 and the others to reconstruct the original folder. Use software like WinRAR, 7-Zip, or The Unarchiver
Ensure you have H-RJ435815.part1.rar , H-RJ435815.part2.rar , and any subsequent parts. Use software like WinRAR