OK, I see now. Judging from your first post I was under the impression that you were hosting on your own server (meaning that you were also the server admin), but that doesn't seem to be the case.
Disk quotas are set up and controlled by the server admins, who in this case would be Surpass Hosting's techs. They're the ones who configured the 3G limit for your account, and although they may have some way for you (or they) to further slice up and manage that 3G, you would really have to speak with them about that. I looked on their support site, but there was nothing I could find there which answered the question of whether or not they would or could do such a thing.
Not having an account with them, I also couldn't look into what user/account management tools are available to you as the account owner. They do seem to use the WebHost Manager (WHM), which has the ability to manage quotas, but A) I don't know if they let you use those features, and B) they're running Linux servers by the looks of it; you would need to be familiar with that operating system in order to manage things like your disk quotas.