I've answered my own question.

According to ProMash, DME contributes 46 points per pound per gallon. That means in 3 gallons of 1.046 wort, there is the equivalent of 3 lbs of DME. Using the SG of 1.046, that wort weighs => 3 gallons x 8.345 lb/gallon (weight of water) x 1.046 = 26.19 lbs. Of that, 3 lbs is DME, so the weight of water is 26.19 - 3 or 23.19 lbs. 23.19 lbs of water x 1 gallon/8.345 lbs = 2.78 gallons. That means the 3 lbs of DME contributed 0.22 gallons, or 0.0733 gallons per pound of DME. 0.0733 gallons equals 9.38 fl oz.