Three point shooting is a function of PER (mainly), Speed, Ball Handling, OFF IQ, and ATH.
If I had to guess, I would put PERimeter about 50% of the equation, Speed 25%, and the other three making up the final quarter.
In a vacuum, I set my guys to +2 as soon as PER is over 70 and their other stats are near 70.
There are always other factors though like:
Do I have another 3 pt shooter?
Do teams play a - defense vs me?
What are the defensive strengths and weakness of my opponent?
Can the guy score using LP/ATH/BH?
What position does he play?
Who will be covering him?