Even though he's slumping, his OBP is still better than the guys you might want to put in there (Plouffe, Arcia). I suppose Suzuki could move up but I don't think that's a great idea. With Mauer's track record, unless he's hurt, you'd assume he'd starting hitting again. Willingham has good on base skills but Morales, Arcia and Plouffe are more power than on-base. Ideally if the Twins had 1-3 hitters getting on base a lot, those 4 guys should get a lot of RBIs.
Is OBP the only criteria for #3 hitter, though?
And at that, Mauer's OBP this year is only marginally better than that of Plouffe and Arcia (and identical to Morales' career OBP). Meanwhile, his power is dramatically less than basically everyone else in our lineup -- like, similar to 2014 Hicks power (or lack thereof). In fact, by OBP and ISO, Mauer is basically an equivalent hitter to Hicks so far this year. (B-Ref's Rbat has them about equal too.)
And I think we are creeping beyond slump territory here -- we're 40% through the season, and he's shown no real signs of recovery. Even if it's not absolutely necessary yet, it would be nice to see early confirmation that we're not going to hang on with him too long at the #3 spot, and that we're not going to let him dictate where and how much he plays forever regardless of his current on-field abilities/performance.