if that 3390 adaptor doesnt have splines, then it is defective - all the 3390 adaptors I have have the splines cut in them. the 3 bolt wheel should bolt to that adaptor, as I had an older Grant wheel with a 3 bolt attachment before I put the turquoise one on Geronimo. Unless they changed the adaptor design recently the 3 bolt will work. The spacer on the wheel is needed for flat faced wheels, as the deep dish wheels will clear with no issues. Don't forget the split pins for the turn signal cancel, and use silicone grease, like spark plug boot grease, to prevent the wiper from wearing the copper disc out on the horn adaptor.