1. The IBM effort under John Cocke. This was virtually unknown outside IBM until much, much later, but may have been the inspiration for IBM projects including ROMP, Power, and PowerPC.
2. The Berkeley RISC project. An academic project that was probably the main inspiration for SPARC.
3. The Stanford project. An academic project that was definitely the main inspiration for the commercial MIPS RISC.
SPARC and (commercial) MIPS were developed at much the same time so it is unlikely that commercial MIPS had much influence on SPARC. The original Stanford chip was a bit older than SPARC, and the SPARC people were certainly aware of it, but in looking at the major differences between Stanford and Berkeley RISC (sliding register window only in Berkeley, and not much else) SPARC follows the Berkeley model.