Starters generally have an OG between 1.03 and 1.04. So if Mr. Malty says you need a 3-liter starter, you'd add between 8.3 and 11 oz of DME to 3l of water, assuming 1.046 points per pound per gallon for the DME.
The Fiesty(sic) Goat Brewery est. 2007 & Clusterfuggle Experimental Ales est. 2009
Primary: Saison, Berliner Weisse, sLambic I, sLambic II, IPA, 70/-
Secondary: Orange Blossom Mead, Flanders Red I
Kegged: Fat Man Porter w/ 1469, Fat Man Porter w/ 005, Centennial Falcon w/ Conan, Centennial Falcon w/ Denny's, Barrel-aged Fat Man Porter, Belgian Dark Strong, Dark Mild