You'll need a MIP for each PS3 to make this work correctly, so basically you'll need 1 public IP per PS3 anyway.
The SSG is a "business-class" firewall, not a home router, and as such things like game consoles aren't really catered to. In order to have multiple gaming consoles behind a single public IP, you have to have a firewall that supports UPnP. The SSG (and SRX) do *not* support UPnP, nor should they.