Hmm.... ranking Horford #3 might be a bit of a stretch, but there have been a lot of names tossed out in this thread that have no business being ranked above him.
Al obviously does not rank well in scoring or rebounding categories. The former because his USG is modest (just 21%, making him firmly just a 3rd option scorer) and the latter because of the way he is deployed, spending a lot of time away from the post on both ends of the floor.
But in pretty much every other type of measurement, Al does extremely well. In particular several 'analytic' stats rate him very highly so far this year. Filtering for guys 6' 9" or taller who have played at least 500 minutes and are predominantly PF or C, Al is currently ranking:
WS/48: 9th
VORP: 10th
BPM: 4th
DBPM: 5th
OBPM: 5th
AST%: 1st
BLK%: 4th
On scoring, even though he hasn't put up a huge volume of points, he's actually been pretty dependable. He's only scored below 10 points (9) in one of his 17 games. And he's scored 17 or more in over half of them.
And I personally blame the one really low-scoring game on Steven's strategy for that game. That was the DET game where we kept trying to post up Al and SVG kept doubling him, forcing him to pass out. So he only got 5 shots in that game.
In the first DET game, we had Al take 8 of his 12 shots outside of 13 feet, which helped pull Drummond out of the paint - and we won. I don't know why Stevens decided it would be a good idea to try to post him up down low in the second DET game.
But I digress. Taken as a whole, Horford's overall numbers stack up pretty well against the other bigs in the NBA so far this year. I don't think I'd put him at #3 though. I'd probably rank him overall somewhere between 5th - 8th.
At some point, it starts to become a matter of whether you like strawberry, chocolate or vanilla ice-cream best. So exact rankings become pretty dubious. I'm comfortable with knowing that we have one of the top 10 PF/C bigs in the game. And I especially like that we have one who's game is so versatile and well-rounded.
I don't think statements about who you would trade him for 'straight up' really answer this question, btw. I would trade Al in a heartbeat for KAT ... but that's because KAT has so much more future value. KAT isn't really a better basketball player right this minute.