If it was 3 teeth off, you would have noticed a BIG change in performance, so it's fairly certain the timing went south the last time it ran (and stopped). Had you cleared the broken serp belt completely out before the last time it ran? It sounds like some of that belt got pinned on the inside of the harmonic balancer and then found its way into the timing cover at the crank pulley (which is the only way in from the outside). And once pieces of serp belt are in the timing loop, it's easy to imagine something getting into one or more of the pulley teeth, causing the belt to jump. So I don't think you can say at this point what the compression really is. If it were mine, a compresson test would be the next step.