Hi,
I'm not aware of any Linux appliance that will provide what you want, there are quite a number that will handle two WAN connections - with load balancing and fail over and you can find details on one of these the "Sophos RED 50"
here.
I am unaware of any that would support three outgoing connections, I'm sure that they are out there it's just that the best person to advise you on this would be a communications specialist.
As to the inward facing connections, you'll need link aggregation enabled at the switch and server level and IPMP along with link aggregation enabled at the server level if you want to make use of this functionality.
Your other option which I think would work, would be to configure a Linux server as a bridge server driving all three of your WAN connections by grouping the interfaces. However I'm not sure how it would work as you'll have one internal IP Address translating to three external IP Addresses. you can find an article and a lot of good information
here.
Regards
Gull04
