How heavy were the rabbits? Say 5 kg? LD50(50% chance of death) of hydrogen cyanide, the toxin in apple seeds, for rabbits is 6 mg/kg. So if they're 5kg then that's 30mg of hydrogen cyanide. 10 grams of appleseed contain about 1 mg of hydrogen cyanide. So you'd need to feed each rabbit about 300 grams of apple seeds. Apple seeds weigh about 6grams per 100, so that's 5000 seeds. Apples have, on average 10 seeds each meaning 500 apples to contain enough cyanide for a 50% chance of killing a rabbit.
To dose 3 rabbits would take 1500 apples worth of seeds.
Have your rabbits eaten 1500 apples recently or did you harvest the seeds from 1500 apples and feed them to your rabbits?