I couldn't even begin to guess how many cups are in 3 lbs. of D.E., but if you go to your local pool supply store they'll probably have a handy little D.E. Scoop which holds 1 lb. and probably costs about $5.
I'm not quite sure I buy the part about only using 1.5 lbs., though. Sure, some D.E. will always be left on the grids after backwashing, but since you don't know how much remains I would rather have a little too much than not enough. When I did service, I was taught to use the full amount after each backwash and then we disassembled the filter and manually cleaned the grids annually. If you get too much D.E. on the grids, then the pressure goes up and the filter cycle decreases. I don't think that's nearly as bad as the alternative, not having enough D.E. on them. I usually don't tend to nay-say a manufacturer's instructions, but that's my 2 cents on the matter.