I'm confident in all 3 of these guys as shooters, and I wouldn't put too much into a summer league 3 pt contest at all.
Fred's offense does not require you to shoot 3's with a lot of guys right in your face. It's all about spacing and getting guys basically wide open looks.
Nobody has as slow of a release on their 3 pt shot as Niang does, and he does just fine. I'm not concerned at all.
As someone else mentioned, last year Matt was just WAY too hesitant sometimes on letting his shot go, and he would pump fake instead of trusting it, and then he wasn't taking his shot in a rhythm like Fred always preaches he wants. He will adjust this year IMO, and trust it.