Multiple WAN lines - source routing

Hi group

I have this challenge.
3 WAN lines on Ethernet.
1 Watchguard x700 firewall.
Need to be able to assign traffic from specific internal networks
(vlan) through specific WAN line.
Our firewall handles one site to site VPN + multiple home users.
I have Vmware as server platform.


I'm looking for a soloution for this setup. Maybe some linux
product(s)
I imagine I will use x700 as gateway for VPN connected to one of the
WAN lines.
Plus some linux firewall + router for handling other lines and
security.


My network must only see one gateway from LAN side.


Best regards
Jesper Harder