I have a secondary drake flock where I've gone/going through similar issues. The drake I brought in from my main flock had been the one being picked on, but now he's become the aggressor towards the old leader, and the other two drakes have joined in as well.
I find a couple dog crates in the coop has helped a lot. Not only do I put one or two of the most aggressive in their crate every night, but they also stay inside an extra hour or so in the morning. So, it not only cuts down on any fighting they'd do at night, but it also gives whoever is on the bottom of the pecking order a chance to eat and have a bit of peace in the morning. You can use the crates to give drakes timeouts during the day too, but I haven't found that to be terribly effective with my drakes.
Like
@BelovedBirds said, multiple feed/water stations help, but I've found that less useful regarding pools. Even when they can't see, if my boys hear any splashing, they'll jump out of one pool and beeline straight for the other to claim it as their own.
Beyond that, I'm not sure what you could besides making sure the 1 duck has enough space to get away from the others.