Ok look at it like this.
Say the guy available at #3 (Buddy Hield, for instance) has a ceiling of JJ Reddick (great sharp shooter who can get you 18 a night) and a floor of Anthony Morrow (quality role player who can hit threes off the bench).
Say Embiid has a ceiling of Hakeem (dominant two way superstar) and a floor of oden (never plays a significant minute).
Would you trade #3 knowing it has a better chance of becoming a decent player... But the guy you are potentially getting has a superstar ceiling with a 50% chance of never playing?
I'm not saying it's as cut and dry as that, but that seems to be the logic behind the idea. Supposedly nobody at #3 has allstar potential let alone superstar potential. Of course, we know flukey things happen so maybe someone taken in the middle of this draft develops into a star. But the pre-draft consensus right now seems to be that your chance of getting a future superstar at 3 is slim to none... Whereas, healthy embiid is still seen as someone who can be a game changer.
Yours is a horrendously flawed analysis.
The first problem is your initial supposition. Do you really think that of all the players available in this draft NONE will ever be better than JJ Reddick (who, BTW, has never averaged 18ppg in a season during his entire career)? That's an awfully bold and frankly presumptuous statement. Do you know how many drafts in the last 15 years had no player selected 3rd or below who wasn't better than Reddick? None. Zero. Zip. Zilch. Yeah, so it's almost certain that there's someone better than Reddick out there.
Your second problem is a tidbit you consistently seem to conveniently forget. Embiid was NEVER considered a sure-fire superstar coming out of the draft. Even coming out there were serious questions about his durability, injury history, his lack of polish (6 fouls per 40 minutes), turnovers, lower body strength, defensive and offensive awareness and even his motivation. His draft position was based on his rare physical gifts and great upside. But he was considered raw, like steak tartare-raw. But because he was so young there was time to work with him. Now here we are two years later. He hasn't played at all. There are still questions about his health. His first contract is halfway done. Why would anyone think his value has increased?
Here's the final flaw in your analysis - the odds are not close to the same between the a productive #3 pick and Embiid's future. The odds of selecting someone equal to or better than Reddick at #3 are fairly high. I went back a few years to get a more complete career track record of #3 picks: Otto Porter, Beal, Kanter, Favors, Harden, OJ Mayo (Westbrook was 4th), Hortford, Morrison, Deron Williams, Ben Gordon, 'Melo. I gotta tell you I like those odds. That's one true bust in Morrison (and consider the GM...). Porter is OK but not exceptional from a truly awful 2013 draft. Mayo is an under-performer. But Kanter, Beal, Favors are all very solid players and are borderline stars. Harden, Hortford, Williams, Gordon and Melo were/are all All-Stars. Of course, Harden and Melo are considered superstars. Were ANY of those guys considered sure-fire superstars when they were drafted? Melo perhaps, but none of the others. Yet teams consistenly found very good players available in that spot.
So if the #3 pick has a 20% chance of a superstar, a 30% chance of getting an All-Star, a 30% chance of getting a solid player, a 10% chance of getting an average player and a 10% chance of a bust....well, those odds are far better than the odds on Embiid. At this stage you'd have to consider Embiid almost a 50/50 whether he ever plays meaningful minutes. That doesn't leave much room for the "superstar" and "All-Star" categories.
Embiid at this point isn't worth the #3 pick in any way, shape or form. Too many unknowns. Too many injuries. Too much learning still left to do. Whereas the #3 historically has been a far more sure-fire thing where All-Stars are
routinely picked.