I just crunched the numbers to see if there's any truth to this. It doesn't look like it, unless you're suggesting there are some geographical anomalies.
I looked at all of the 64 teams (didn't bother with the first four) and what they shot cumulatively for the season from deep versus what they shot during March Madness.
Overall, the teams shot a collective 35.36% from three during the season (exclusive of MM), and 34.43% during March Madness. It's been awhile since I took stats but I assume increased level of competition has something to do with the marginally lower percentage.
I also looked at it team by team. We're shooting 8.6% below our 3P average (28.1% vs 36.7%) and have the highest deviation from the mean of any team that made it to the Elite 8. The next closest is Duke, that shot it 5.3% below their 3P average (33.3% vs 38.6%).
Alabama, of teams that made the E8, is at the other end of the spectrum. They're shooting 41.4% in the tournament, which is 4.9% better than their 36.5% season average.