Download all three source parts into one directory.
macOS/Linux:
cat sources.part-01 sources.part-02 sources.part-03 > ffmpeg-core-0.12.10-sources.tar.gz
shasum -a 256 ffmpeg-core-0.12.10-sources.tar.gz

Expected archive SHA256: 755f319d1ced059f8900af94d20a9cf3a61fbbe738186044e2caefe611549b22

Windows PowerShell:
$output = [System.IO.File]::Create('ffmpeg-core-0.12.10-sources.tar.gz')
foreach ($name in @('sources.part-01','sources.part-02','sources.part-03')) { $input = [System.IO.File]::OpenRead($name); $input.CopyTo($output); $input.Dispose() }
$output.Dispose()
Get-FileHash ffmpeg-core-0.12.10-sources.tar.gz -Algorithm SHA256

Then extract the archive and read README.md and SHA256SUMS. Original source licenses and build instructions are included.