Yes. You can catch One of the Latis after beating the Elite four. Also, if you obtain Eon or Mystic tickets You can get Deoxys an Mew...I think
In Emerald it is possible to get Rayquaza, Groudon, Kyogre, Regirock, Registeel, Regice and Latias or Latios. It is also possible to get Mew, Lugia, Ho-oh and Deoxys (Speed form) if you get some special tickets.
For Pokemon ruby you can catch the three regis, rayquaza, groudon, and latios/latias. In sapphire you can catch the three regis, rayquaza, kyogre, and latios/latias. However, in emerald you can catch kyogre, groudon, rayquaza, the three regis, and latios/latias.
Here are all the legendaries you can get without cheats: Groudon, Kyogre, Regice, Regirock, Registeel, Rayquaza, Latios/Latias.
groudon,kyogre,rayquaza
There are many legendary Pokemon in emerald, such as groudon, rayquaza, groudon, deoxys, and such. Groudon and Kyogre are not available until after beating Wallace, but Rayquaza can be caught right after the major battle between groudon and kyogre.
No, you need to get Ruby. Or you can get Emerald where you can get both Kyogre and Groudon, including Rayquaza.
You can't get Kyogre in Pokemon Ruby but you can get Kyogre in Pokemon emerald and sapphire and in ruby you only get groudon and rayquaza. In sapphire you get Kyogre and rayquaza and in emerald you can get groudon, Kyogre, and Rayquaza.
Rayquaza, Kyogre, and Groudon
It is in the sky pillar
kyogre rayquaza and groudon
You can catch Groudon, Kyogre, Rayquaza, Regirock, Registeel, Regiice, Mew, Latios or Latias (you get to choose) and Deoxys (only though special events)! You can only catch these Pokémon after you've defeated the Elite Four and the Hoenn League Champion!
the Pokemon emerald legendarys are: groudon kyogre rayquaza latias/latios deoxys (with aurora ticket) mew (with old sea chart) jirachi is possible to get but i don't know how lugia (mystic ticket needed) ho-oh (mystic ticket needed)