Crabapples are widely variable with respect to sweetness, acidity and astringency. However most often they are very tart and astringent. You might want to cut back to like 1 lb to limit these effects. I wouldn't use more than about 2 lb unless these are very sweet and tasty crabapples, which is entirely possible. Can you eat these? Or are they unpalatable? If not good eaters, I'd cut back to 1 lb, or maybe 1.5 lb max.
My 2 cents.