It makes at best 1-2C difference at normal RPM (<1500). U14S doesn't exactly need more static pressure because it's thin&spare heatsink (compared to wide & dense heatsinks like U12A or NH-D15).
Additional fans only increase pressure, they don't increase volume of air. You probably already have exhaust fan at back of the case that is few cm distance from U14S, there is no need to have 3 fans in row.
Yes some people mount even high-pressure industrial fans to NH-U14s but it's really bad idea... those kind of fans belong to super-dense heatsinks like Ice Giant ProSiphon or water-cooling radiators.
I run my 3990X with U14S and single-fan at only 800 RPM to make it silent. But 3960X/3970X do benefit from faster speeds.
Threadrippers, or Ryzen CPUs, also don't suffer from lack of heatsink dissipation, but from thermal transfer between the chip and the cooler. Even million fans will not help with temperature much because the chip is hot under the heatsink, the heatsink is ok (just try to touch it, it's not super hot at all).
The only way to lower temperature is to lower the room ambient temperature. It's good idea to work in <22C in general anyway. Or reduce voltage through power thermal limit.
Overheating Threadripper simply downclocks to lower frequency, it doesn't cause any crashes. Over-heating memory... can cause crashes. Those are hard to cool but they actually easily run 70+C on Threadripper boards with XMP profiles in 1.35+ Volts. Good room ambient and strong case airflow are important then.