I'd recommend a "rolling respawn."
You can set an area trigger on the CVBG to fire a remove trigger on the first S-3 and then fire a add object trigger for new S-3's on the way.
There is also a doctrine level command called TacticCenter, that might also be available in the mission editor scripts, try looking for it and see if you can script it for the S-3 based on a refiring area trigger for the CVBG, that would be a more elegant solution, but it might not be available, of course.
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LW

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