Answer:
You can't get Celebi in Emerald you get Jirachi after you get 200 Pokemon on your pokedex he will be roaming the region and as for latias and latios if you picked red or blue in your house the one you pick will be roaming the region. hope this helped.
You can't get jirachi or celebi in Pokemon emerald but they're therios about them but they're not true they're just people using cheat codes.
As for latios/ latias, you can find them after you beat the elite four. You'll be in your house in your room after you beat the elite four and you'll watch the TV and your mom'll ask you a question about whether the Pokemon in the TV is blue or red. If you choose blue, you can find latios. If you choose red, you can find latias. Then go to lilycove city with a tropius that knows sweet scent, a woffubet lv 27-30 (lv 27 is the best), 20 ultra balls and 10 timer balls, and 10-15 repels. Use a repel and go into the grass and use sweet scent. I did this trick and i found latios on my second try. tell woffubet to use mirror coat and hope that latios uses luster purge because then mirror coat leaves with 1-10 hp. Then throw ultra balls at it and you should be able to catch it probably your 5th or 10th try. AND REMEBER TO SAVE BEFORE LOOKING FOR LATIOS AND REMBER THAT LATIOS/LATIAS RUNS AWAY! ALSO IF LATIOS/LATIAS RUNS AWAY, LOOK UP LATIOS/LATIAS IN THE POKEDEX (latios #381, latias #380) AND YOU CAN SEE WHERE. IF LATIOS RUNs AWAY, STAY IN LILYCOVE city and keep using sweet scent in the grass andin 5 to 10 minutes latios/latias will be back in lilycove city. and don't fly or run to latios/latias or use bike it'll run away!
2nd and 3rd paragraph by lalagato.