The difference between an extra 300 and a delta wing plane are night and day, though. I think your plane will handle just fine at 3kg. With something like the extra300 or a cessna you've got a clear difference between the lift creating wings and the fuselage.
Designs like the XR-2 and DeltaGlider take real advantage of the lifting bodies concept, getting additional lift from the fuselage. If you look at our F-117 model, its got very narrow wings with a length of 84cm and a wingspan of only 55cm, only the last 10cm or so on each end actually being "wing"! Still, this model handles 560g(20 oz) no problem because the fuselage itself acts like a big airfoil.
Keeping that in mind, you have a loosely airfoil shaped body that will probably help more than you think. We can talk about aeronautics and physics all day long if you want, but I have found that the laws of flight are tamed down when you are working at the speeds and wing loadings we work at.
One last note, make sure you maiden this puppy in the nose heavy direction. Delta planes quickly turn into a fun, short show for your spectators when they are tail heavy. Generally, a tail heavy plane will take off, flip, and die. A plane that is too nose heavy will simply roll off the end of the runway. Just make sure you don't have to apply too much up elevator to lift off, or you'll have a comet.
Good luck, can't wait to see this project done.
