i have 3 adsl lines each line 1 mb
so, i want to merge it and balancing the download and upload also (if one or two lines from my three lines were cut i want the internet is on from the line that is on)..
otherwise, you could use load balancing to send each packet through a different gateway (could break some programs that use many connections), or use policy routing, to send different kinds of traffic through different gateway. for example WWW through one, VoIP through other and rest through third.