If you look at the 2005-2015 drafts, Mayo's had a better career than 5 of the 10 #3 overall picks. Could be 6 if Bortles doesn't keep on an upward trajectory, I marked him as undecided. From 2005-2015, the following guys were drafted at #3 overall:
Worse than Mayo: Braylon Edwards, Vince Young, Tyson Jackson, Trent Richardson, Dion Jordan,
Better than Mayo: Joe Thomas, Matt Ryan, Gerald McCoy, Marcell Dareus,
TBD: Blake Bortles
#3 picks may be blue-chip prospects, but that doesn't make them great pros. The bust rate is much higher than you seem to be suggesting. Upon inspection, Mayo would've been perfectly in line with what you should reasonably expect out of the #3 pick.