A Pelican 1510 rolling carry-on case.
http://www.pelican-case.com/15caonca.html
You'd have to pluck out the foam to create the slots, but it isn't very hard -- the foam is scored so you can rip it out one one-inch block at a time. I've gone through the plucking process for three cases and haven't screwed it up yet.
The 1510 is almost certainly big enough for three trumpets but Pelican doesn't make a smaller case that's long enough for a standard Bb trumpet. Tough to beat for $130, though.