You do know that the DraftExpress mock is a guess based on incomplete information, whereas the actual draft is the more accurate reflection of how teams valued the players involved ... right?
No he read blogs and there is a clear consensus that says that isn't true. Everybody knows that
Pick a side and stick with it. Did the media suggest Boston messed up by taking Brown over Dunn? Did we hear reports that several teams wanted Dunn and selecting the 8th best prospect over him ruined Boston's ability to make a trade?
Or... was the difference between Dunn, Brown, Chriss, Bender, Hield and Murray completely negligible... meaning that Boston reaching for Brown at #3 didn't matter, because everyone acknowledged the guys in the 3-8 range were interchangeable prospects?
Or are you suggesting that everything being reported in mock drafts, blogs, articles, podcasts, media personalities, etc was entirely wrong... and that all teams had Brown going 3rd in spite of literally every piece of information available to the ignorant public?
Pick a reality and stick with it. If your belief is that everyone we read was wrong and Brown was the consensus #3 pick amongst all teams, so be it. Let's build that conspiracy and start making each other tin hats as a show of support.
If, however, you want to go off the version of reality that was available to us as fans - leading up to the draft there was no consensus which of these prospects was best. Many had Boston taking Chriss 3rd. Many had Jaylen Brown going 8th. In the hours leading up the draft, Kris Dunn started gaining traction as a possible consensus 3rd - he ended up going 5th. Some felt Bender should have gone 3rd - some felt he should have gone 8th... he went 4th. Some were arguing Buddy Hield should be taken 3rd. Some felt Hield should go 8th. Hield went 6th. There was no consensus whatsoever on which of those guys was the better prospect.
There was, however, a major consensus that every team picking outside the top 2 was reportedly trying to trade down or out of the draft, because of an acceptance that there was no consensus pick available. Ultimately it just came down to throwing darts at a board and gambling on guys they liked slightly more. I suspect Jaylen Brown would not have been our pick if we weren't already loaded with guards. It's pretty clear a major reason we took Brown was because selecting him wouldn't cause too much of a roster log jam. Selecting Dunn, would have been an issue since we already have Bradley, Thomas, Smart, Rozier, etc fighting for guard minutes. Selecting a SF made more sense.
A different team selecting 3rd probably doesn't take Brown. That's an important note to make. That's where Lord Ford's Word-of-God Draft Tiers come into play. When you're selecting from a group of guys in the same prospect tier, you end up selecting the one that makes the most sense for your team. All the guys expected to go 3-8 ... went 3-8 in some order. After the higher tier guys (Simmons and Ingram) were off the board, these were the 6 interchangeable lesser prospects available to choose form.