Looking at dozen spins or column spins you´ll soon discover than balance might be far away. Only one part may hit the average - and the other two may come MORE AND LESS. From time to time you´ll see Permanenzen in which the less frequent dozen or column appears closer to 1/4 than 1/3 of the spins.
I have had some luck with a method on single numbers. I set a certain goal, say a certain pattern inside each column. After a hit I leave the column. After leaving TWO of the three columns I´ll write the notes from the remaining column in a new piece of paper and start anew with blank spaces in the other columns. I´ll reach my goal in two of the three columns and reuse the notes from the left column in the following session.