Although I hate to disagree with Lynn, I would not recommend willy-nilly use of 3MM apex seals, especially in a non-turbo engine.  3MM ceramic seals have excellent detonation resistance valuable in a turbo engine, but regular 3MM seals are only slightly stronger than the 2MM.  However, the 3MM seals supposedly do not seal as well at high RPM (I don't have any real evidence of that, but I've heard it from several trusted sources) and tend to chatter more, increasing wear on both the seals and the rotor housings.  Mazda switched from 3MM to 2MM seals because the 2MM seals were better overall.  The strength of the seal is just barely relevant in a non-turbo engine.  Since aircraft engines spend almost all their time at high RPM, the 3MM seals are particularly disadvantaged (and chatter wear is evident in most disassembled aircraft application rotaries).