As far as I noticed, total count of pillars should be less some %%, like 50% (I'm not sure about exact value).
If you have more - only pillars will be on the wheels, if less - pillars will never be on wheels.
So, put as many "good" rares as you have, and fill with pillars to 30 cards.
6 (rare1) + 6 (rare2) + 6(rare3) = 18 and 12 pillars, you most probably will only have 3 rares on wheels and quite big chances of winning one.