I'll be curious how this works out for you. When I had a full-faced motorcycle helmet (on my motorcycle, not bicycle ) I would frequently have trouble with it fogging up in cold weather (sub 30°F.) However the 3/4 may have enough ventilation at the bottom to prevent it. If it doesn't work, it will be because the steam and moisture from your breath moves upward and has nowhere to go but the backside of the shield in front of your eyes.

I finally solved most of my fogging/icing problems by switching to ski goggles. Still happens, but only when the temp is well below zero and usually doesn't get bad enough to cause me issues before I get to work.