Six wheel -three axle cars as best I can tell:

First 6 wheel/3 axle circuit racing car to compete in a major series=Pat Clancy Special Kurtis Offenhauser in the 1948-1949 AAA Championship=2-4 layout=Driven by Billy Devore, Jackie Holmes and Mack Hellings=Best finish - 4th in the 1949 Milwaukee 100 by Jackie Holmes

Last 6 wheel/3 axle circuit racing car to compete in a major series=Tyrrell P34 Cosworth in the 1976-1977 Formula 1 World Championship=4-2 layout=Driven by Jody Scheckter, Patrick Depailler and Ronnie Peterson=Best finish - 1st in the 1976 Swedish Grand Prix by Jody Scheckter