I don't have a specialized catalogue, but can find no variety of this stamp in SG with a "Three" overprint. Not sure what you have there. The mirror image of the "Three" suggests that perhaps it's an offset from the paper on which it was mounted or something like that. I'll leave it at that and maybe someone with more knowledge can comment.
Looking at it closer, it does appear that the "THREE" is on top of the gum- and there is a faint red outline- It is certainly possible that the print was transferred from what ever it was laying on- a nice transfer if that is the cause.
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