Not sure how well Zen Darm is going to fare in a 3v3 format. Most of Zen Darm's power comes from setting up on switches from opponents fearing a hit from Gorilla Tactics Darm and sweeping from there. In a 3v3 format like online laddering, there will be less opportunities for Darm to set up. I'd say Scarf Gorilla Tactics is probably going to put in more work.
In 6v6 formats, Darm appreciates slow Volt Switch/U-Turn users to get him in safely. Rotom-W is a great partner, checking opponents like Gyarados and Sand Rush Excadrill, spreading Burns, giving Darmanitan more potential set up opportunities, and offering a relatively slow Volt Switch to get Darm onto the field. If Darmanitan isn't holding Heavy Duty Boots, it also wants hazard control from the likes of Excadrill, Corviknight, etc. Screens support can also be useful to make setup easier. Lastly, Psychic Terrain support from Indeedee can stop priority users from picking him off after a Belly Drum.