Actually, the NeXTStep style was adopted by Windows 3.1 and that made it look subtly different from 3.0. Given the timing, I think the actual direct influence on Windows 3.1 was Motif, which in turn was inspired by NeXTStep.

When the NeXT was announced I thought it had been influenced by Randy Smith's (Xerox Parc) ARK (Alternate Reality Kit):

http://worrydream.com/refs/Smith%20-%20Experiences%20With%20...

But it might just have been a coincidence.