Well, you can get that trio with virizion and cobalion and terrakion, i got them all but i kinda forget how i did, but i know where you should go, you can find them any time (porobably when youve beat the game of course) so terrakion is fight and rock type, you can find it by exploring the caves and tunnels at victory road. virizion can be found by crossing the double bridges and going through that open part in the fence, on the right side. go though there and you will end up in a place with lots of tall grass and lots of small ledges, get past and pass by the little pond go north (i think) and just find your way around to the little dark shadow (you know like it tells you there is another foresty room) go through it and virzion will be in the clearing! by the way i think you need to find cobalion before terrakion and virizion i dont know for sure. anyways, you need to go on route 6 with a pokemon that knows the HM surf, what you want to do is when you just leave driftveil city and there is a small bridge, use surf on the opposite side of the rocks. then you will be at another small bridge. keep going north and then go to the right. be careful not to go to far so i recomend sticking to the right side of the river to see it. anyways, there is an old man by the lab who tells you about a legendary pokemon in the cave. as i was saying, go onto the right side land and there will be a cave! you can also find axew here.so look around and you will find him. there is also kyuerum, the ice and dragon type. you find kyuerum by going to the giant chasm, go in the cave and reach the very bottom and go into the light (you get there by going all the way right then like 3 steps up and then go left all the way and u find stairs that go down to the light) so go there and you will find a place with a fair amount of trees and lots of dark and normal tall grass. you can find ditto here and metagross and other pokemon, what you want to do is get to very center of the giant chasm, its like a maze, the very center is a small pond, you can look up how to get there becuase i forgot but i have done it before. while your in the giant chasm's back, you'll see that there are steps on a side that you cant get to because there are trees in the way, so if you follow the line you can obviously see one of those things that you can only get across a center way (kind of like a leadge thingy? i dont know what to call it) so anyways, once you reach the center (if your volume is up) you will hear a pokemon cry then everything will get white, when the screen gets normal again it will be snowing everywhere, it covers the tall grass and the trees, so you can go all the way to the top right corner and up the stairs and enter the cave! in there you will find kyurem!
(i strongly advise that you save the game right before you battle the pokemon because if you make it faint you can just turn of your DS and then back on and u can fight it again. i hope this helped, just soemthing cool for you to know, in case you accidently beat volcarona the first time (when team plasma tries to steal it) you can get one again! it will just take a little longer... your pokemon will REALLY need to know surf for this one. go to route 1, you know the little stairs that leads down to a river? well now surf on it, follow the river and you will still be on route 1 but the pokemon on the next bit of land you get on will be in the 30's! isnt that weird? anyways, just get past the hills and stuff and go to the very bottom left corner, u will find one of those route gates, go through it and you will find... ROUTE 17!!!!! so, once your there, surf your way through and get past that swimming girl, and go to the land. up the stairs and im not so sure after that.. just look around for a small blue house, becuase there will be a ranger (or some person) in there who will give you an egg... nope. it wont be a volcarona. IT WILL BE A LARVESTA! this is the only way to get larvesta (besides catching the volcarona and makeing it have a baby) so that happend to me and i got the larvesta and now its like lvl 51 and it evolves at 60 i think into volcarona. i think i got all the legendaries that you can find any time. (I AM SO MAD BECUASE I MISS ALL THE EVENTS FOR VICTINI AND THOSE POKEMON AND GENSECT AND IM SO MAD) so i hope that answers your question