Tim McAlester. Hit a game winner at Syracuse in 84 (don't remember if it would have been a 3 had it existed at the time) and at home in 87 against NC State.
David Little would be another contender.
Tim McCallister had a great game at Syracuse, but didn't hit a game-winner. I was at the game, so remember it quite vividly. I believe the final score was 98-91. T-Mac had 26, and Wayman led the way with 33. Every time Syracuse made a run, though, T-Mac hit an outside jump shot to settle the crowd. He would be one of the top-5 3-point shooters had this existed throughout his four years.
Another thing about T-Mac. He was no worse than the 2nd best outside shooter as a Sooner freshman in the last half-century. He just didn't have the sustained improvement as did Buddy Hield or Hollis Price.
Here are the 5 best pure shooters from my point-of-view:
1) Buddy Hield
2) Hollis Price
3) Brent Price
4) Tim McCallister
5) Trae Young
It is weird have Trae Young at #5 considering he has ridiculous range and may already the 2nd best NBA Sooner of all time despite only four seasons as a pro. However, he's more streaky than the four guys listed ahead of him.
Also, I'm looking at this as "Who would win a 3-point contest" if they were all in their prime.