(I'm assuming you want to use Eternatus, Ditto and Snorlax...)
I'd first like to mention that Eternatus seems like a very niche anti-meta pick for series 10. However, one issue is that it still gets obliterated by Zacian. The issue with specs Eternatus is that you'll be locked into say, sludge bomb and Zacian switches in...
+1 252 Atk Zacian-Crowned Behemoth Blade vs. 0 HP / 0 Def Eternatus: 163-193 (75.8 - 89.7%) -- guaranteed 2HKO
The issue with using Ditto is that you will always be having to take 50/50s. A large majority of the game comes down to whether or not you get to transform into Zacian or Xerneas.
Because of the aforementioned calc, alongside the fact that without Specs, Eternatus can not OHKO Zacian, I decided that it would be better to use a cosmic power set Eternatus.
+1 252 Atk Zacian-Crowned Behemoth Blade vs. +1 20 HP / 156+ Def Eternatus: 85-102 (38.9 - 46.7%) -- guaranteed 3HKO
252 Atk Zacian-Crowned Behemoth Blade vs. +1 20 HP / 156+ Def Eternatus: 58-69 (26.6 - 31.6%) -- guaranteed 4HKO
252+ SpA Kyogre Ice Beam vs. +1 20 HP / 172 SpD Eternatus: 74-88 (33.9 - 40.3%) -- guaranteed 3HKO
What this means is that if you can set up a single cosmic power, you can potentially become unkillable whereas Ditto is your secondary win-con.
The Eternatus set can be adjusted to suit yourself but:
Eternatus @ Black Sludge
EVs: 20 HP / 156 Def / 4 SpA / 172 SpD / 156 Spe
IVs: 0 Atk
- Cosmic Power
I've already explained some of the calcs but here's another one:
252 SpA Life Orb Calyrex-Shadow Expanding Force (120 BP) vs. +1 20 HP / 172 SpD Eternatus in Psychic Terrain: 179-213 (82.1 - 97.7%) -- guaranteed 2HKO
Ditto is your secondary win-con and the set should be reasonably self-explanatory:
Ditto @ Choice Scarf
EVs: 252 HP / 4 Def / 252 Spe
Transforming into Scarfed Urshifu could still at least allow you to hit Zacian before it OHKOs you etc. I'd argue Naive is better over Docile.
Incineroar feels mandatory in this series due to the prevalence of Zacian and physical attackers. Additionally, fake out support pressure pairs really well alongside Eternatus, allowing it to set up a cosmic power for free. AV Incineroar feels a bit questionable, I would personally run AV Rillaboom and Sitrus Incineroar but if there is a very justifiable reason behind your decision, you can stick to it. I would also suggest increasing Incineroar's speed to outspeed other fake out users.
I would genuinely reconsider Snorlax due to the fact that Urshifus will be incredibly dominant in the series 10 meta. However, a positive behind using Snorlax is that it rounds up your matchup against Calyrex-S and allows you to lead it against Xerneas and get off 40-50% (assuming they don't lead intimidate, where it would become 26-33%). The spread that you are using is a bit outdated because it was mainly calced to live series 9 threats instead of series 10 threats. Yawn will no longer be necessary due to Togekiss so I would recommend protect.
Rillaboom feels necessary to round out your Kyogre match up and I definitely agree with it. Your spread specifically lives ice beam from Kyogre and Behemoth Blade from Zacian so you should be able to wood hammer Kyogre confidently in front of Tsareena. I feel like protect is unnecessary but I understand that it is mainly a counter-preventative measure to the prevalence of fake-out in series 10. However, I would definitely change high horsepower to wood hammer because you already have it on Snorlax and it still doesn't greatly benefit your Zacian match up. Another niche in opting for Rillaboom is that it provides additional passive healing for Eternatus, allowing it to set up more cosmic powers and less recovering.
Lastly, because of your huge weakness to Urshifu, I felt like Togekiss was mandatory. You can also use this to set up cosmic powers turn 1 and yawn is a great asset.
Togekiss @ Babiri Berry
Ability: Super Luck
EVs: 188 HP / 76 Def / 4 SpA / 236 SpD / 4 Spe
IVs: 0 Atk
- Dazzling Gleam
- Follow Me
The spread specifically lives Behemoth Blade from Zacian and Thunderbolt from Magnet Regieleki:
252 SpA Magnet Transistor Regieleki Thunderbolt vs. 188 HP / 236+ SpD Togekiss: 152-182 (82.6 - 98.9%) -- guaranteed 2HKO (You can adjust it so that you have more HP and less SpD and live with 6.3% chance).
+1 252 Atk Zacian-Crowned Behemoth Blade vs. 188 HP / 76 Def Babiri Berry Togekiss: 151-178 (82 - 96.7%) -- guaranteed 2HKO
Overall, I feel like I haven't given many recommendations or changes aside from swapping Urshifu for Togekiss which I felt was necessary. Your newly adjusted team should have a more passive playstyle involving using Eternatus as a win-con and Ditto as your gamble win-con in case Eternatus goes down. Snorlax is another potentially sweeper once Urshifus are removed from the field, being able to set up curses against bad match ups. The meta hasn't been established yet and I haven't familiarized myself entirely with series 10 yet either. However, I can only offer some recommendations:
Gothitelle: Trapping passive enemies with Eternatus can allow it to automatically stack up.
Celesteela: A secondary win-con who has interesting type synergy with Eternatus.
The main thing is to test the team and trial and error but you can take some of my words into consideration, apologies for I don't really have too much to say lol.