Read in his book a while back that best indicator for a young 3yo to get a classic distance is impressive efforts at a mile while attending the pace. Makes sense as its a distance that combines speed and stamina, especially if run around 1 turn.
Races won on or near the front end at a 1 1/8 (a distance that inherently favors forwardly placed horses) are less impressive than at a mile or 1 1/16th.

Good Luck,
Joe B