Your DPS could really go any way, but I'd say melee DPS are inferior to ranged or magic. They just get killed the fastest in battle and are then useless. I agree you probably want a mage or illusionist, they're just invaluable in hard dungeons. This would be the best person to specialize in more than one class, though.
Inferior is the wrong word. Melee DPS provide some of the highest damage output out of any class, more than Ranged (an optimal geared MA/Thief easily outdamages an optimal geared Ranger/Hunter. It's awakened bow vs awakened dagger + nearly 30-40% more attack). The only time I get killed in battle is really with reflect, which is something all classes are weak to (casters with insta cast fall prey too, while ranged is the least effected due to its attacks being generally weak). The argument here is melees are closer to the boss thus get killed easier. This is why having resistances and LA's higher def helps. On the flip side, bosses have inescapable AoEs that effect all, making resistances on certain bosses required for everyone. There are some bosses where a melee isn't best suited for, and that's really the only time I ever go ranged. All other times, I choose melee DPS.
To the OP,
Level all the classes, for certs and class score (gives hp/mp/attack/mattack). Almost each role can use similar if not the same gear. If you want, designate someone with a "role" not a class. Sometimes you have bosses weak to certain elements or physical hits. I wouldn't worry about it at your level, explore the classes. I used to HATE MAs, only leveled it for samurai which I happened to love. When awakening weapons fell into my hands, I didn't expect MAs to be extremely strong, nor Thieves (another class I hated) being strong either. Thus, my most used classes are the Light armor DPS.
Overall, I doubt you will only play with 3 people forever. I used to main tank because my best friend and gf play heal/dps respectively. My guild eventually got a main tanker and now I can go wild with my DPS classes. So keep that in mind, especially when you and your friends find classes in other branches that you like