If you're having issues registering MASH 3.0 having previously had 2.x then it's likely the problem is due to have more than one version of MASH loaded in Maya's Plug-in Manager.
You can have more than one version of MASH installed on a machine but you cannot have more than one loaded in Maya at the same time.
In Maya's Plug-in Manager (Window > Settings/Preferences > Plug-in Manager) make sure you only have the 3.0 plug-in loaded (mash.mll should have installed to \My Documents\maya\20xx-x64\plug-ins). Restart Maya. Then try to register again.
You can have 2.2.3 registered alongside 3.0 (you just can't have them loaded in Maya at the same time).