an enormous amount, enough to feed a hundred people.*
The Church Fathers, as you might expect, were not about to leave that "three measures" unexegeted. The Catena Aurea includes quotations interpreting the flour as simply "a great abundance," but also:
"the whole world"
"the whole soul"
"those three things in man, with the whole heart, with the whole soul, with the whole mind"
"the three degrees of fruitfulness, the hundred-fold, the sixty-fold, the thirty-fold"
"those three kinds of men, Noah, Daniel, and Job"
"spirit, soul, and body"
"reason, anger, and desire"
"the Law, the Prophets, and the Gospels"
"belief in the Father, the Son, and the Holy Spirit; which when it has fermented into one lump, brings us not to a threefold God, but to the knowledge of one Divinity. This is a pious interpretation..."
So while what I'd thought was the basic meaning of the parable -- that a small amount of Christians can leaven the whole world -- is attested in the Fathers, but so too is a meaning on a more modest scale -- that a small amount of Christianity can leaven the whole Christian.
There is clearly work to be done in leavening the whole world. And if the whole soul is not yet leavened, grab another pinch of the kingdom of God and mix it throughout all three measures.
* This sort of thing, we are told, was a real knee-slapper in Jesus' day. "Three measures of wheat flour? Why, that's nearly four and a half pecks! How tired the woman's arms would have been! Landsakes, is that ever amusing in its hyperbole and ridiculous in its exaggeration!"