I think what people are saying is that there may be enough information for you to fix what you are looking to fix with the information you have already provided.
Perhaps you need to tell us exactly what your issue is so that we can help you. Sounds like you trying to find a way to split packets that create a new connection mark 3 ways instead of 2 ways, which would be one obvious way of fixing your problem.
Alternatively you should be able to assign a set number of users to a set gateway and split your three adsl lines that way. But you haven't told us specifically what it is that you require of us. That means we are unable to specifically tell you what to do i.e. 30 users on your network get assigned to 3 groups, each group uses roughly the same bandwidth as each other and you assign a group to each adsl.
I can't help as I'm a MT newbie. But if you clarify your question I'm sure someone will be able to help.