3 - Nothing at all.

No need to use grease. It all slips on easy enough. Some coolant that may be still on the fittings will help with slipping on. You shouldn't be 'preventing rust' if your using coolant and not just plain water.

No need for any sealant. Good hose, good surface as long as its not coroded will seal fine with a good clamp. I'd see using any sealant as a band aid fix.