I used to tape the Inkbird probe to the outside of my keg, then wrap that area with a folded wash rag to help insulate the probe from the air inside the fridge and got good results.
Also depending on the volume of beer involved, it can take several hours or perhaps a day or more to effect several degrees of temperature change.
I used to brew outside with water temps as high as 76 F in the hottest days of summer. After I cooled the wort down to about 85F with an immersion coil, I would put it the fridge with my temp set to 64 F. It would take about 24 hours or more for 5 gal to cool down to 64 F. Then I would pitch my dry yeast.
If your temp readings are consistently 3 degrees off after two or three days, you can adjust the amount of deviation as Minbari said, or simply add 3 degrees to your initial setting.