I would recommend gyarados for the third Pokemon in the trio.
Since this is gen lll, Hyper Beam is physical and that's really good for gyarados
Rain Dance is to power up Waterfall, and Dragon Dance is to get faster and to power up Hyper Beam.
Gyarados is also easy to find, but in only one location in emerald, and that place is sootopolis city. If you use the super rod, you should get it in no time. They also are found at a moderately high level(35-45).
Hope I helped!