You shouldn't need to uninstall 2.3. The installer will install the v3 files as per a new install and it's only necessary to specify the existing 2.3 database at the 'Set up data repository' step. It will then upgrade the database to the v3 schema.
I think that the installer showing a port conflict is potentially a bug. The most likely workaround for this is to stop the web service (named 'SQL Monitor 2 Web Service') in the windows services management tool (services.msc) prior to running the installer.
Hope this helps