How do I convert a semi-auto AR-15 into a 3 shot burst?

Converting a semi-automatic AR-15 into a three-shot burst configuration is illegal in the United States under the National Firearms Act, as it constitutes a modification to create a machine gun. Attempting such a conversion can result in severe legal penalties, including fines and imprisonment. If you're interested in a burst fire capability, consider purchasing a legally compliant firearm designed for that function or researching alternative options that adhere to firearm laws. Always prioritize safety and legality in firearm modifications.