Could I possibly get in on this action?
Hehe, just kidding.
To answer you properly, I would surely need more info on how you are managing these separate 3G cards.
The best way i can think of managing them all at once, is to contact Vodacom and setup your own private, corporate, access point.
These separate devices can then be configured to connect to that access point, instead of the usual Vodacom "Internet" access point.
By using this centralized access point, not only will you cut costs, but I believe it will also give you a certain amount of centralized control and management, as well as monitoring, of these separate devices.
You're welcome to ask more questions about it and other options if u like, I will surely assist as much as i can, as often as i can. Please bear in mind though, with the high costs of internet, me being formally unemployed, and having a huge decline in internet business atm i use my connection very sparingly.