when did u transplant to the 3-gal pots? are they rootbound?

use a small amount of dolomite lime to raise the pH just a tad. as bigtops mentioned, 6 is a bit low..

although a 3 gal pot drying out in 48 hours is definitely possible, it seems a bit fast. id make sure that even the very bottom of the pot is dry before your next water.

i recently had the same problem as you (similar symptoms at least) and as soon as i did all these things my problems disappeared. hope this helps..

-pskunk