You cannot fire a three, four, or any round burst unless the weapon is auto. The AR’s I see are semi auto and thus single shot. Yes, I recall the conversion kits from the various mags and trade pubs.
You cannot convert an AR into burst mode without the auto-sear, M16 bolt carrier, and the the selector group. Further, in most cases (but not all, depending on the lower specs) you have to machine the lower receiver deeper in the fire control area to accept those components. HTere are a few holes that need to be drilled as well and unless you have a jig to caaomplish this, it’s not that easy.
Further, the fire control group and the sear are NFA items that cannot be purchased without a valid permit.