Not all American Standard toilets are the same, assuming that all yours
are the same model, unless you bought them at the same time.

2 years ago, I purchased an A/S Cadet model - have had no problem with
it - and recently purchased another A/S Cadet for installation in the
powder room.

When I unpacked it, I found it to be different in design that the first
one I got 2 years ago.

Both look the same, but the newer one has a different size opening
outlet (larger) when viewed looking into the bowl.

On the underside of the bowl, the trapway outlet is the same size, but
the inside of the bowl is completely different in design.

I'm thinking that American Standard is constantly redesigning - even the
same model line - to improve flush characteristics.

Perhaps, although you say all 3 of your toilets are "exactly the same"
... the problem toilet is not as recent an aquisition as the others?

Or ... you may have a crack in the trapway - not necessarily visible -
and that will play havoc with its' ability to siphon properly.

Get an new one - they're not that expensive - I paid $127 for my Cadet,
bowl and tank complete.

BTW - the new Cadet I just installed will suck _you_ into the bowl if
you stand too close while flushing.

Joe