Yeah, don't think this was an equal 1/3 + 1/3 + 1/3 partnership, but in various written histories there is Apple engineering involvement:
"So, with the goal of maintaining RS/6000 software compatibility, a team of architects from IBM, Apple, and Motorola set out to refine the architecture
...
IBM and Motorola, with Apple engineering participation, have put into operation a new design center to develop future PowerPC microprocessors. The Somerset Design Center is a 37,000 square-foot facility located in Austin, Texas, staffed primarily by Motorola and IBM with approximately 300 engineering professionals. The design center is presently working concurrently on three separate PowerPC microprocessors." (https://www.thefreelibrary.com/History+of+the+PowerPC+archit...)
The intro to PowerPC Architecture book includes the following:
"We would like to acknowledge Keith Diefendorff, Ron Hochsprung, Rich
Oehler, and John Sell for providing the technical leadership that made it
possible for the group of architects, programmers, and designers from
Apple, Motorola, and IBM to produce an architecture that met the goals
established by the alliance these companies formed.
Many people contributed to the definition of the architecture, and it is
not practical to name each of them here. However, a core group worked
long hours over an extended period contributing ideas, evaluating
options, debating costs and benefits of each proposal, and working
together toward the goal of establishing a competitive architecture for the
member companies of the alliance. This group of dedicated professionals
included Richard Arndt, Roger Bailey, Al Chang, Barry Dorfman, Greg
Grohoski, Randy Groves, Bill Hay, Marty Hopkins, Jim Kahle, Chin-
Cheng Kau, Cathy May, Chuck Moore, Bill Moyer, John Muhich, Brett
Olsson, John O'Quin, Mark Rogers, Tom Sartorius, Mike Shebanow,
Ed Silha, Rick Simpson, Hank Warren, Lynn West, Andy Wottreng, and
Mike Yamamura."
