How to use 3 ISP Lines (2 mbps each) in one subnet for a 3 levels Company by specifying each level to a certain Line ??

you can use a router for analyz your3 networks and bridge this networks to1 network with6 MB per second bandwidth and control that on your network . you can use mikrotik routers for this . this router is not expensive and you can have3 input internet network with2 MB Bandwidth on3 ports and give1 Network with6 MB Bandwidth on your Output network . Just you need input rout commands and config your mikrotik device .