There are three openings above the venturi of each carb which by rotating the venturi you can closed. My question is: should all the openings be open or is this another way of fine tuning the carbs?

 

By "venturi" do you mean the black rubber intake trumpets? If so, then IMHO no, the trumpets are not supposed to be an adjustment and are supposed to be clear of the holes.

 

From what I have been able to find, these holes are to allow keep the float bowl chamber open to normal atmospheric pressure. So when the Intake trumpets "venturi" compress the air lowering the air pressure that this causes the gas to flow up into the chambers above.

So I suppose that there wouldn't be any reason to close those off.