Appraisal regulations clearly state the cut off for bedroom count is 2 when researching comparables. In other words, valid comps for a 3bed home are to have 3 or more bedrooms. 1 or 2bed comps are ok for subject properties with 1 or 2 bedrooms, but not valid for subject properties with 3 or more bedrooms. Also comps with 3 or more bedrooms are not valid comps for subject properties with 1 or 2 bedrooms.
If you have an appraisal where the subject property has 3 bedrooms, and there are comps with 2 bedrooms, those comps are not valid and violate appraisal regulations (USPAP), and FNMA, HUD/FHA, and lender guidelines and requirements. The appraisal would be considered misleading and possibly appraisal fraud.
And yes, there is an industry formula for determining adjustments for differences of such amenities (in valid comps), whether it be bedrooms, baths, garages, pools or other differences. It is called matched pair analysis and takes in consideration market reaction to such differences and every market area is different in the monetary or value reaction of buyers.