727 3rd gear feels more like a down shift

When the car slows down, it may be cuz the tranny is trying to engage two gears at once, namely second and third. This is not supposed to be possible, so one of them is out of time. Either the second is late releasing, or third is early engaging. IMO, this would have to be a mechanical issue, broken slip-rings, or a busted VB or even a shattered second gear servo piston. Honestly I don't know how second could stay engaged.
Oh maybe I do;
If the servo retainer system failed and the servo popped out far enough to get cocked ... oh wait that's not it either, then it would also suck power big time in first gear, as two gears would still be engaged, but second and first this time.
Nope, I'm stumped.
But the air-pressure test should help get to the bottom of it.