I was saying to pull out 2-4 quarts of weak wort, heat it up (brief boil) adding the 2# or 3# of light DME to it, cooling that, and then pouring it back into the primary. You'll still be at close to 5 gallons total volume (hence the brief boil, to avoid losing too much volume) and it would be more of a 5 gallon version of you 3 gallon recipe...
OR, you could just let it ride, bottle it up, drink it down, and chock it up to experience building... After all, it WILL be beer, just not what you had intended it to be... You could also follow the other recommendation to reserve it to mix in with a heavier/bigger beer later... Although, that would mean leaving it in your fermenter until you've actually brewed the other beer and it's ready for blending. I've not blended anything yet, so I won't give any advice on how much to blend of what. I think it's done more to taste, so you should use something graduated, making notes of how many parts of each you blend, see how it tastes, and go from there.