Hi there, and welcome!
Sorry to hear about your ducks fighting. Unfortunately, it's very common for boy ducks to fight when there are not enough females. To prevent this you really only have a few options:
1. You could get more females (highly recommended!
). You'll need about three or four females per male. They may still fight, but they are much less likely to injure each other if there are more than enough females to go around.
2. You could find a new home for one of the drakes (my second choice). You can find new owners through craigslist or even here on BYC. If you don't have a large space for your ducks, then this is your easiest and probably best choice.
3. You could build a pen to keep one of the drakes in, separate from the other two ducks. Your drake will be very lonely in this case, and you'll probably need at least one more duck (a female!) in order to make him less frustrated and unhappy. And it's a lot of work to build a separate predator-proof enclosure.
Sorry--I know none of those options is easy. But some things just can't be fixed, and a male duck's territorial/mating instinct is one of those. You just have to work with it, and the solutions above are tried and true and will do the trick for you (possibly with tweaking along the way). Good luck!