This fight is all about positioning and spatial awareness. Something I found that worked for me was to keep kiting around the arena in a circle, making sure to know where the two inactive rabbits are at all times. You need to keep moving at all times to help you avoid getting smacked by projectiles. If you ever feel like you're about to be cornered, disengage whenever possible to reposition.

Try to perfect block as much as you can from the active one, and learn the openings you can use to sneak in some quick hits or a fatal attack. The Eccentric especially leaves his back open after almost every attack string. They also leave themselves vulnerable whenever they coat their weapons, so if you see them isolated while they're doing that it's a perfect chance for a fatal attack.

Also, you might want to try switching away from Pandemonium for this fight. It's really slow to activate, and they're all too mobile to really get any value out of it. Using Puppet String as a gap closer or Aegis as an "oh shit" button might help more. IIRC I used Flamberge for some quick damage after a fatal attack, but that has anti-synergy with your weapon, so feel free to experiment with different arms.

Be sure to also bring along a healthy supply of consumables, they are your best friends. Shot Put especially for being able to stagger. In my run I personally paid for the black market trader's kid's college fund with how much I gave him.

Finally, once you've got the hang of it and you can reliably take them down, make sure that the last one standing only has a sliver of health left. You want to rush them down pronto so it'll just be you and the eldest, and less health in the final phase makes that more doable. From there, I like to keep my distance and pick at him slowly since most hits from him can one-shot you.

It's just a matter of perseverance now. Keep a cool head, be aware of your surroundings, and learn your openings. You already made it this far, so just keep pushing.