In WIndows VST3 are instaled on a fixed path C/ ProgramFiles/ Common FIles/ VST3.
VST2 are installed on the path set during the install process.
The plugins you install is also shown during the install process.
The rest is a Sonar/ DAW question.
I believe Sonar do support both VST2 and VST3.
1. Be aware of the bit versions of programs and plugin versions you use. Especially if that can cause read issues.
2. Some DAWs can use VST3 and VST2 version of the same plug-in, and some DAWs can only use one version at a time.
3. You have to tell Sonar/ a DAW the path/s you are using for your plugins, to make it able to read and use the plugins. A path set to a VST3 version perhaps can block for using a VST2 version. Then perhaps you need to move the VST3 version from the path, or remove the path totally if you don't wan't to use VST3.
4. Also the DAW can have scanning you perhaps need to handle if it block plugins. Perhaps you need to delete a blacklist and re-scan, if something gone wrong during the scanning. As said i belieeve you should be able to use VST3 in Sonar if you wish to. I don't know why you are adviced to use VST2.
This is about VST3:
https://www.steinberg.net/en/company/technologies/vst3.htmlBut yes there can be reasons to use one version from another.
But how to do all this in Sonar i suggest your look in Sonar manual and ask in Sonar forum or Sonar support. You can be lucky someone in this forum use Sonar.
Perhaps you can find articles and/or videos on the web that can help you.
I don't think this issue has to do with Arturias plugins.