Hello, welcome to EE! :3
First off, allow me to explain a bit about these classes, so that you know what they do.
Cleric - amazing at healing. If built right, can be good dps as well.
Shaman - mostly used for healing, but not desired as much as cleric. If used to dps, it would be single-target only.
Warlock - single target dps and not used in pve much except for certain bosses that break to dark (Boses 101: if you want them to drop their rare items, you have to "break" them b4 killing them by hitting them with whatever element or weapon type that they are weak against).
So essentially, you would be able to heal, single target dps, and possibly aoe dps with the cleric with this setup.
If you want to be a jack of all trades for magic, I would suggest maining mage and illusionist. Mage is aoe dps, whereas illu is more single target. Both are desired in parties however. The only trade off is that neither of those classes heal. The nice thing about the magic dps classes is that you can generally use the same weapons and armor for all of them. So, you could excel at dps even if you played multiple classes (but you would have to level them all).
In the end, there are many benefits to trying out all of the classes (even the non magic/healing ones), so I suggest that you play around a bit to see what each class has to offer. Class changing is one of the best things about EE after all :3