S3 is a block-based overlay on top of Amazon S3, whereas S3N/S3A are not.
These are are object-based. The difference between S3 and S3N/S3A is that S3 is a block-based overlay on top of Amazon S3, while S3N or the S3A are not because of them being more object based.
S3A is the successor to S3N. The difference between S3N and S3A is that S3N supports objects up to 5GB in size, while S3A supports objects within 5TB and it has much higher performance.