Your main soul is whichever one you put the most points in. It might be the left. It might be the center. It might be the right. The game doesn't rearrage them in order of which is your main, nor does it force you to put more points into one or another dependant on where they are located on the soul tree page.
When/if I get around to playing (damn money is always getting in the way), I will have a Champ/Paragon/Riftblade dual-wield dps build, a Champ/Paragon/Riftblade dual-wield pvp build, a Champ/Riftblade/Paragon 2hand dps build that I might occasionally switch out for a Champ/Paragon/Riftblade 2hand pvp build, and a Paladin/Warlord/Void Knight tanking build that might trade in Reaver instead of Void Knight depending on how I'm feeling. Gotta cover all those bases, you know.
Pro tip - Rogues, learn how to play Bard and have a build in mind for one of your roles. Warriors, learn how to tank(or dps conversely) and have a build in mind for one of your roles. Mages, learn how to .. Chloromance?, and have a build in mind for one of your roles. Clerics, learn how to dps, and ... well you get the point. The beauty of RIFT is that every calling is flexible, and in dungeons that flexibility will be put to the test on many occasions.