Can you boil all three gallons? If yes, add all extract at once. If not - if you're boiling less than the full volume - start the boil with the DME and add the LME at flameout, before chilling.
I'd also recommend adding a half-pound of 10L Crystal or so, just for increased mouthfeel and a bit of residual sweetness. Depending, of course, on the brand of extract you've chosen.