Do I understand correctly that you are thinking of ONE case to hold three basses?
If I wanted to ship 3 basses, I'd go directly to Anvil or Calzone and have them make me a custom case with rollers. And then I'd get ready to pay the airlines an over-weight charge every time I shipped it, because there's no way it would be under 50 pounds. Most of the airlines either already charge, or are about to begin charging, extra for bags over 50 pounds.
And incidentally, I'd insure them through an insurance company specializing in musical instruments. Given your profile, I suspect you already have such insurance.
In my former life as a videographer, I often had Calzone in Texas build me custom hardshell cases for air flights. They are a bit lighter than anvil cases, which made them more manageable at the destination.
http://www.calzonecase.com/
But if you're a serious enough professional musician that you truly need three basses to travel, it's worth it. If you're not a pro, then you probably don't need three basses when you travel...or you may be able to ship them to your destination ahead of time in individual cases.