Does it have a little hole drilled above the safety selector, with a small pin holding in a little chunk of metal and spring called an auto sear? If so, it's an illegal, unregistered machinegun receiver conversion. There's no way to make it a legal semi-auto, and it must be destroyed or surrendered to the police or ATF.
If it's got a drop-in auto sear, but is otherwise an unmodified semi-auto receiver, you can remove and destroy or surrender the drop-in sear to make it a legal semi-auto.
The full auto parts must come out. You can't keep it tri-burst or auto legally.
If it's not an illegal receiver, then depending on what parts are installed, you'll need to grind down the existing parts to semi-auto specs, or replace them with semi-auto parts. The parts to be modified or replaced are: selector, hammer, trigger, disconnector and bolt carrier. The tri-burst has a second "burst" disconnector which must be modified as well.