Y3, YN3, B3 normally means that the flight is at best full and very likely oversold. They are willing to sell up to 3 additional high-fare seats, but that's not an indication of free seats - just an indication that they are willing to oversell it a bit more and VDB/IBD people if needed (with a fare of $434, they still come out in front!)

It used to be that "M" was the fare class you needed for SDC, which was basically an indication that the flight wasn't oversold, but I'm not sure if that's still the case.