I get this, but the extra 30-35 PAs the 3 hitter gets over the 5 hitter seemingly would outweigh this, and if the 3 guys also gets on base a lot more than the 5 guys, he will set the table for the next guys to get more rbis.
Maybe part of the reason the #5 gets more rbi opportunities is because you put a great hitter up before him (in the 3 slot).
I'm not discounting the studies, but those 30 or so extra PAs help more than just for rbis.