This paper introduces the new version of the Madeleine portable multi-protocol communication library. Madeleine version III now includes full, flexible multi-cluster support associated to a redesigned version of the transparent multi-network message forwarding mechanism. Madeleine III works together with a new configuration management module to handle a wide panel of network-heterogeneous multi-cluster configurations. The integration of a new topology information system allows programmers of parallel computing applications to build highly optimized distributed algorithms on top of the transparent multi-network communication system provided by Madeleine III's virtual networks. The preliminary experiments we conducted regarding the new virtual network capabilities of Madeleine III showed interesting results with an asymptotic bandwidth of 43 MB/s over a virtual link made of a SISCI/SCI and a BIP/Myrinet physical link.