Havent tried that but just did some measurement with vents covered with a sticky tape.
Red is uncovered, green is all 3 covered.
Pretty interesting, and might prove useful as i do find them sometimes to have too much bass for my taste. Doesnt look that it affects the rest of the FR, though my measuring rig isnt perfect, YET :).
Will try at a later point what changes would covering 1 or 2 holes have.