The 3 gun rules on the website for the OKC Gun Club say .308 win (7.62 x 51 NATO) or larger. 300 blk makes major rifle for USPSA Multigun, but I believe the OKCGC 3 Gun matches follow a modified version of IMG rules which require .308 win or larger.
USPSA major PF and the division of heavy metal are different animals. I also believe the PF for HM is prohibitevly high for intermediate cartridges. IIRC HM requires a PF of 360.