I think shots bigs and guards take are not the same. Do Kelly's defenders stick to him as close as IT's are? Hardly. Plus an irregular shooter like Smart arguably would fare better in a 3PT competition than in game. That being said bigs have obviously better percentages.
That's a special case because IT is an all-star and a 20+ PPG scorer. Defenders stick to Bradley as well, but he gets a lot of his shots off the dribble so that makes sense.
I think those two guys are the best shooters on the team for that reason - they have the toughest shots (many are off the dribble and contested) and still shoot a good percentage.
But I think Olynyk, Horford, Jerebko are certainly better three point shooters the Smart and perhaps even Crowder - neither of the latter gets a huge amount of defensive attention when at the three point line.
Rozier might be a different story - he's not shot well in the NBA so far, but he's shot decent numbers in summer league and in the d-league, and even half decent in college. He looks to have decent form, and a lot of his shots are off the dribble, so not easy shots. I think he could become a good shooter.