[updated]
If you were to deploy a more recent version (e.g. the current Balance 380 or 305) then you could get up to 1 Gbps, assuming you could balance your ISPs equally. With your three 200 Mbps feeds you could achieve 600 Mbps if your traffic was in the form of separate concurrent sessions that the balancing algorithms could spread equally across the WANs.
even if the device could support a throughput of 600mbps, you can’t simply join three separate WANs to get 600mbps on a single download stream. you could balance though. speedfusion does allow bonding but there is overhead.