My friend had exactly the same issue in his Jackson just a couple of days ago. He also had only two springs in it and I told him to get the third in there. He bought the extra spring and I did the setup and it came out fine I think.
The adjusting screws in the cavity had to be set of course.
We also adjusted the hight of the bridge.

BTW he's had .009's all the time. He just changed the strings and the bridge jumped up mysteriously although
the string gauge stayed the same.

[Edited by janir123 on 04-09-2003 at 02:47 AM]