The Thieves do add up, one at a time, so 3 Thieves do not yield 45% stolen, but the 39% previously mentioned. In my last game, I put 2 with a Navigator in one Sub and a Pirate with a Navigator in another Sub. With a Diplomat in range, I could hit Outposts protected by Sentries with the 1st Sub, the Diplomat would free it, then I would target it with the 2nd Sub. Wash, rinse, repeat.