Re: having another ACDF and scared!
Kvolf - There's been a long debate about "Adjacent Disk Syndrome", where fusing one level puts more pressure on adjacent levels and causes them to break down. It is not clear how much of a concern this may be. Studies have shown that the percentage of adjacent-level problems that need surgery are low enough (and the time they take to develop is long enough) that it could be argued that what looks like causation is really more a matter of correlation... that is, that adjacent levels break down FOR THE SAME REASON as the inner level, not BECAUSE the inner level broke down.
Whatever side of this argument you take, the fact that two levels broke down only two years after another was fused, that it's happening to someone so young, and that all the levels herniated (rather than just bulged) means that something unusual is going on. Either your disks are, for some reason, very weak, or there is (or was, at some time) an unusual amount of pressure on them.
I'm sorry that I can't offer a brighter prognosis. I hope that you can get a handle on what the problem is. Of course - you never know - the disk degeneration could stop at this point and never trouble you again.