Aggression from males towards prospective mates is normal amongst more reptiles. The problem is that in captivity, a female unwilling to mate can be bullied endlessly and has practically nowhere to go but in the tank. So I suppose your female really did not want to mate but your male was persistent enough to bite her leg of. You really need to keep the turtles separated at this point, and you need to schedule a visit to the vet.
Meanwhile, you need to make sure there is no bleeding. Your turtle may be in shock too. To slow down bleeding, cool the turtle down. If you are unable to visit a vet for now, keep the turtle on a cool dry towel for the first 24-48 hours. You need to clean the wound once the turtle calms down and the bleeding stops. Clean the wound with some betadine and clean water. From then you can place the turtle in clean shallow water and give it the opportunity to bask. Or you can dry dock the turtle, but be sure to place the turtle in water for some time and offer food daily. The turtle may reject to eat and that's OK just keep trying until she eats. If there is an infection, oozing, or puss, contact a veterinarian. Not doing so will cause septicemia, or blood poisoning. If not treated quickly this can kill your turtle. Aggressive antibiotic treatment is required to cure septicemia, and only your vet can offer that.
Keep cleaning the wound frequently and leaving the turtle in its state until you're sure the wound is healed. It is way more preferable to visit a vet, but if you are entirely unable to do so then these procedures can help you save your turtle. I recommend keeping the male alone, and if you really need to put them together, provide a hiding place the male can't access, and monitor very closely. If you notice persistent aggression again then you need to separate them for good. The recommended male to female ratio is at least 1:3. The more females per male the better. If both females don't want to mate with your male he will resort to hurtful aggression, so more females will ensure your male won't exhaust a particular female, and hurt her like he did now.
Hope your turtle heals, please tell me how it goes in the future.