Close. Each thief goes in order, though, not simultaneously.
So the first thief would steal 15.
The second thief would steal 13 (.15x85 remaining, rounded up)
The third thief would steal 11 (.15x72 remaining, rounded up).
Total stolen = 39.
So, without kings or generals interfering, the 30 driller sub + 3 thieves against a 100 driller sub:
69 drillers v. 61 drillers +10 shield = outpost defended successfully, with 2 drillers left.
"No man chooses evil because it is evil; he only mistakes it for happiness, the good he seeks."
-- Mary Wollstonecraft