If you said they were healthy with roots coming out of cube and were planted in that cup 3 days ago in that soul they could have grown a lot and are already starting to fill out the cups, have had that happen before on some beefy plants and they started dropping because the roots needed more space,, looks like overwatering but a slightly bigger pot can compensate for that,, I would water one more time, then wait till there almost dry and just tip the cup over and let it slide out a little, if you see any roots, any,, you should transplant