You can put vinegar on them or some of the products to keep kitties from biting them. However, three weeks is a long time for skin stitches. When did the vet tell you to come back and have them out? She probably does not need them to hold her skin together any more, and they bother her.
I would be concerned that her stitches have not been removed after 3 weeks which is why she is picking at them. When are they suppose to be removed? The stitches themselves have likely picked up a lot of bacteria by now and could cause an infection.