Of course Joe's SLG has been higher than league avg for 3rd place hitters in nearly all of his seasons. 2011 Being a glaring exception.
Just an FYI - I've been doing a lot of research for an article I've been writing, which is basically about Joe Mauer vs. the rest of the #3 hitters in baseball (He's not nearly as good in this regard as you think) that I may or may not ever finish. For this, I have a spread sheet with all the primary #3 hitters for each team in the league last year.
The results of that show your statement is flat out not true (and I assume is the same for almost all other years). The "average" slugging percentage of primary #3 hitters last year was .489, Joe Mauer was .446. "Average" amount of HR's was 23.03, Mauer had 10.
And If I do a little more math instead of taking a straight "average", the slugging percentage of primary #3 hitters across the league last year was .491.
Now, he does come out above average in a lot of other categories, but he's not nearly the same level of run producer (which a #3 hitter should be) as most others.
And since you brought up slugging percentage, the only primary #3 hitters Mauer beat last year in this category were: Justin Upton, Jason Kipnis, Brett Wallace, Alex Rodriguez, Chase Utley, and Kyle Seager. That's only 6 out of the 29 other players.
And thanks Nick for agreeing with me about "best roster" going north and how 6 years down the line shouldn't come into the thought process!
Scouting Report: Power: 30, Hitting: 50, Arm: 60, Defense: 40, Speed: 40. "Line drive swing and shows good contact and on-base abilities. Double's power at his peak. Strong arm from 2B or the OF, stiff hands. Not a fast runner, but above average instincts on the bases. Skinny body doesn't look the part, but can sneak up on you. ACL surgery sapped much of his athleticism." (Probably)