I'm going to echo what a few others have said. I've been following Bradley for a long time and it will definitely NOT be a 'miracle' for him to shoot above 35% from the 3PT line.
At every level where he's gotten significant minutes, he's been able to pour in shots from outside.
College: 29.5 mpg, 42 of 112 -> 37.5%
D-League: 32.2 mpg, 11 of 27 -> 37.0%
Celtics (2011): 5.2 mpg, 0 for 5 -> 0%
Celtics (2012): 21.4 mpg, 22 of 54 -> 40.7%
He was also an excellent shooter in H.S. and scouts frequently have cited his excellent catch-and-shoot form.
Taken all together (almost 200 attempts) that becomes a compelling sample size that suggests his 'true' percentage lies somewhere between 35 and 40%. Of course, he's still young and likely to improve his shot.
He turns 22 this coming November.