Yes no one knows until its tried. Usually if a phone states up to 32GB SD card, its because 64GB SD cards weren't released at the time of publishing the specs*.
There is no "Limit" on disk size that an SD card slot can handle*. The only restrictions are the file system and since 32GB is the usual cut off point for FAT32 but teh S3 already surpasses this, I cannot think of a technical reason why the S3 would not handle 128GB. That said, no one knows until tried. Its always a risk
*Exceptions to this were the difference between SD and SDHC as there were physical differences but that's legacy now.