Installing dotnet30 from either a standard script or as an installed component will fail with a MD5 checksum error. This appears to be something that is easily fixed by assigning the correct MD5 sum value.
Also still having this issue, it is specifically with .NET 3.0, or if you try to install 4.5 then the same error occurs when it gets to 3.0 in the installation. 4.0 seems to work fine as it does not attempt to install 3.0.
Related: https://www.playonlinux.com/en/issue-5413.html and https://www.playonlinux.com/en/topic-14903.html