The furnace is in the attic. I call for heat. The furnace makes a loud noise, then shuts off. All while happening a red LED light flashes in sequence 3 times. What do I do from here? It says open pressure switch on the troubleshooting chart.

The loud noise could be a disconnected blade on the inducer striking the housing. Code 3 also means either your inducer fan isn't working, the vent is plugged, the switch is bad or plugged or the tube from the inducer is plugged, leaking or disconnected.