How many litres/gallons in a bucket? if its just a normal 10L/2-3G one then times it by three buckets, thats 30L/7G or so, 29G total water volume is 120 ish litres, 30L out of 120L is about 25%, so drop the water level about 1/4 of the way down in the big tanks.
Made sense to me anyway
EDIT: by the way, it doesnt have to be any perfect amount, just drop the water level a third or so of the way down and you will be sweet.