Despite the fact that "below" and "under" can have the same meaning, these two phrases are actually quite different.
I have 2 swimming qualifications under my belt = I have achieved 2 swimming qualifications.
That move was below the belt = The move was underhand/illegal/unfair.
I have a feeling that in the case of your specific example, it may be that the tailor has sold his soul to the devil in exchange for 3 extra inches of length to his genitalia. I'm not certain as there's no actual context but if I had to guess, that's what I'd go for.
Edit: I've just read it again and had another thought. If he's a tailor then he's dealing with material. If he's making trousers then there would be an area of material physically below the area where the belt goes, so it's possible that the "selling of his soul to the devil" is metaphorical and he literally has gained three inches of material to the piece of clothing.