That’s it. Install HALion 6 and the HALion 6 content (so that you have all the presets in HALion 6) - you will finish up with usable installations of HALion Sonic 3 and HALion 6 with all the content and no wasted disc space because there is no duplicate installed content. As HALion Sonic 3 remains installed, your existing projects using HALion Sonic 3 will continue to work.
You should always install the full installers including any content packs for any Steinberg software newly installed on a system. Steinberg Library Manager is intelligent enough not to reinstall the same version of the same content, so you will likely find the content install for HALion 6 is very quick because you already have HALion Sonic 3 installed.