Only Studio One Pro can use third party VSTs, or you can buy a VST add-on for artist. Beside "Instrument Library" you you will see another tab called "VST plugins".
Regarding your plugins folder, the VST3 folder is only for VST3 files, don't put you're DLLs there. The usual path for 64 bit plugins C/Program Files/VST Plugins, or C/Program FIles/Steinberg/VST Plugins. Some VST installers will make this folder for you. Some simpler installers will just give you a DLL to put in this folder. If you don't already have this folder you can just make it.
32 Bit plugins will go into C/program files (x86)/VST Plugins (or C/Program Files (x86)/Steinberg/VST Plugins), but Studio One won't use 32 bit plugins and there's no need for them anymore, but you can keep them there if you want.
VST3 is a newer format, it's hardwired to install to C/Program Files/Common Files/VST3. If your plugin has a VST3 version it will install here. In the Presonus browser you will the see two versions of a plugin (VST2 and VST3), you can just hide the VST2 version.