The CDS has a section for % of class with GPA below 3.5. However, some colleges use weighted/recalculated GPA or do not fill out this section at all. The specific numbers for Tulane, as listed in their 2018-19 CDS are below. Admit rate is clearly below the OP’s 30% threshold, and a good portion of the class is below 3.5. Tulane is far from the only school that meets this description.
Admit Rate = 17%
Mean GPA = 3.56
65% Below 3.75
38% Below 3.5
19% Below 3.25
7% Below 3.0
1% Below 2.5
With the extreme variation between different sub 30% admit rate colleges, it’s difficult to give a generalized answer. At many low admit rate colleges, few students have less than a 3.5 UW. At colleges where few have a less than 3.5 UW, there is often a good reason that those few sub 3.5 UW applicants are admitted, some of which have been listed in this thread. With a more specific list of colleges and more specific detail about the rest of the application besides just UW GPA, I could give a more specific response.