So, I did some research. Since 2002, here is how each class of Rutgers has shot from 3, collectively. So the "freshman" stat counts everybody's freshman year.
Freshmen: 32.39% (2291 attempts)
Sophomores: 32.74% (2153 attempts)
Juniors: 33.84% (2568 attempts)
Seniors: 32.42% (1903 atempts)
Now compare us to Villanova (I picked them sort of at random, but also because they've had one coach this whole time and fewer guys leaving early than Duke, Kentucky, etc, and because in my mind I have this idea of their guys improving throughout their careers).
Freshmen: 34.13% (2098 attempts)
Sophomores: 35.10% (3311 attempts)
Juniors: 36.91% (3541 attempts)
Seniors: 37.22% (2990 attempts)
Now, there are a couple of caveats because Jay Wright is recruiting a higher-caliber player, and he focuses specifically on guards, but... the difference in improvement is striking.
Let's look at two Villanova era, the Foye-Ray-Sumpter era and the Arcidiacono-Hart-Jenkins era.
3P% by year, freshman to senior, attempts in parentheses:
Randy Foye: 30.2% (96) ... 29.9% (144) ... 34.0% (153) ... 35.0% (254)
Allan Ray: 29.9% (127) ... 39.2% (222) ... 37.6% (213) ... 37.2 (288)
Curtis Sumpter: 23.7% (38) ... 31.8% (66) ... 43.3% (90) ... 36.6% (142)
Ryan Arcidiacono: 32.7% (217) ... 34.5% (171) ... 37.2% (148) ... 39.4% (193)
Josh Hart: 31.3% (83) ... 46.4% (112) ... 35.7% (154) ... 40.4% (183)
Kris Jenkins: 37.0% (81) ... 37.2% (129) ... 38.6% (259) ... 36.0% (239)
So except for Jenkins, who basically shot the same throughout, everybody else gained at least 5 percentage points on their three-pointers, with a few seasons of a 10+ percentage point improvement.
Now let's look at the Carter-Mack-Seagers era:
Eli Carter: 35.3% (156) ... (32.0% (122) ... (transferred, injured) ... 30.5% (141) ... 27.9% (229)
Myles Mack: 33.8% (142) ... 46.2% (132) ... 35.9% (181) ... 32.4% (185)
Jerome Seagers: 33.3% (87) ... 38.1% (84) ... 41.6% (89) ... (transferred) ... 34.7% (124)
A bit more bouncy, to say the least.
What about current guys?
Corey Sanders: 31.5% (130) ... 26.9% (78) ... 21.2% (80)
Mike Williams: 24.0% (100) ... 32.0% (150) ... 29.9% (127) ... 24.1% (54)
Issa Thiam: 30.1% (93) ... 39.8% (98) ...
So Corey's regressed from an already-poor 31% and while Mike improved 25% to his sophomore year, he started too low for it to be a "good" number.
But imagine if Sanders' numbers had been like Williams'. 31.5% his freshman year, and then a 25% improvement would've put him at 39.4%! And then even if he fell off seven percent (as Williams did from SO to JR), he'd be at 36.6%.
What's my point? I don't really know. I lost track a while ago. But improvement from 3-point shooters is absolutely possible. Among the many other shortcomings of the program over the last 15+ years, this complete lack of development is is yet another reason we've remained in the basement.