Can I ask you a question

[MStowastiqVahlshdeh]

Ok does this mean it is a good idea to go for multi leveling as long as I keep my two other jobs below my main one?

As long as you progressed through the storyline enough, if you look under System> Active Help archive and select Armory Bonus, it will explain how the armory bonus works.

Even if you do not have pop up Active Help text set to active, it will automatically archive every help page that you have accessed so far in the game.

Armory bonus help info comes up the moment you switch to your first cross class, if I recall correctly. I know its only use able on DOW DOM classes. You get maximum bonus by leveling the class that is the farthest level from your highest leveled class.
From the manual:

"To lessen the burden of leveling for multiple Disciple of War and Magic classes, an experience point bonus known as the Armoury bonus is added when slaying enemies and participating in FATES. The bonus grows larger the greater the discrepancy between your current level and the level of your highest class. Use this to your advantage when deciding when and how to raise secondary classes."

Also you get rested bonus which also adds experience points bonus. It is from being in Sanctuary areas or in your Inn Room and you will see the moon icon on your exp bar when you are in a sanctuary and accumulating resting bonus. It accumulates while you are offline, if you log out in a sanctuary.

Using Food also gives you a 3% exp bonus.
Good Luck To You