LME has a potential of 36 ppg. That means 36 gravity point for every lb. of LME in a gal. of water. So, 3.3 lb. will give you about 119 gravity points. On average, base malt at 75% efficiency will give you about 25 gravity points. So, it would take right around 5 lb. of grain to get the same gravity as 3.3 lb. of LME. Again, this assumes you get 75% efficiency when you mash the grain. when you actually do it, you should take careful measurements and see what your own efficiency really is so you can correct the next time if necessary.