To get Entei, Suicine and Raikou, you must have the following things. You need to have a Legendary Pokémon on your team and go to the Mirage Spot, Trackless Forest. This is located to the east of Rustburo City. Once there, enter the portal and catch the one that appears. After you catch that one, remove the legendary Pokémon from your team to access the portal again.
For Cobalion, etc. You will nneef 3 Pokemon with full EV's. Then, near Pacifidog Town, you will see Pathless Plain. Land there. Cobalion will appear on Sunday, Wednesday, and Friday. Terrakion on Tuesday and Saturday, and Virizion on Monday and Thursday. Also, as these are time based, you can't change your time to get them to appear.
So, yes, you can get all 3 pokémon in those trios.
In case you were curious about the other trios as well, here you go.
For Uxie, etc. You will need 3 Pokemon with max happiness. Next, fly to Nameless Cavern near Sootopolis City. Interact with the portal. Keep in mind that these are time based as well, Mespirit is 4:00 AM to 7:59 AM, Uxie is 8 PM to 9 PM and Azelf is 9PM to 3:59 AM.
Now, for Dialga and Palkia. You can only catch Dialga unfortunately. However, to catch Dialga you will need Mespirit, Uxie and Azelf in your party. Fly to a year in the sky and there is Dialga. Then you will need to trade for Palkia to get Giratina. You will need to place Palkia and Dialga in your party and then fly to the same spot.
For Reshiram and Zekrom, you will need a level 100 Pokémon in your party. Then fly to Fabled Cave by Route 110. Interact with the portal. Then done. If you happen to KO it, defeat the Elite 4 again. Also, you will have to trade for Reshiram, as it isn't available in your game. For Kyurem, you will need BOTH Reshiram and Zekrom for Kyurem, then fly near Mt. Chimney.
https://www.wikihow.com/Catch-Reshiram-and-Zekrom-in-Pokémon-Omega-Ruby-and-Alpha-Sapphire#:~:text= Capturing Reshiram or Zekrom 1 Damage,it. Once the Reshiram or Zekrom's... More
I included these because I wasn't sure, but if you don't need them, so be it. Hope I helped!