Both 2 and 3 will happen.....these types of movies don't necessarily need to follow quickly behind the first movie...it has its base fans fully intact, and as soon as it begins to be promoted.......the quality of the first will be remembered, and the seats will be filled again.....
I don't think that "Grind House" will have any affect on SC2 and 3.....GH just didn't have the fan base, nor the headliners to bring people to the theatre to see it.....the quality of SC will be remembered and the people will flow in as they did before.