General aviation pilot reported an engine malfunction during the descent on a training flight. The instructor took control and landed safely at the destination airport.

Narrative

The event began as we were inbound to ZZZ via billboard. Our route of flight was ZZZ-ZZZ1-ZZZ2-ZZZ. During the entire flight the weather conditions were very good. We had set up a VNAV profile to be at 2500 by the time we reached billboard; & when we reached around 4500 ft the engine began running very rough. We recognized this to be abnormal and consulted the engine roughness checklist. We determined that individually selecting the left magneto led to worse vibrations so we turned that one off and operated on the right magneto. Around this time while checking engine instruments we saw that cylinder 3 had an EGT of 0. My CFI decided we needed to [request priority assistance] and they contacted approach to let them know of our situation. We requested assistance with getting to the nearest runway which ended up being XXR; and had traffic stay clear of the area. We were able to keep our descent to around 250 FPM as we were concerned about losing too much altitude before reaching the airport. I continued to try to ensure checklists were being completed while my CFI handled the controls. I don't remember if it was like this the whole time; but at one point I recall that the power was set to full on the one magneto and only at 2100 RPM. We were able to make it to the runway and though we came in fast; we touched down safely and no harm was brought to any of us onboard. My CFI did a very good job coordinating with approach as well as letting XXX know what was going on as we were inbound. During the entirety of the flight; we encountered no abnormalities or issues that I would point out to causing this event. Preflight inspections came out fine as well; during the magneto check in the runup I remember calling out a maximum drop of 100 RPM and a maximum difference of 10-20 RPM between magnetos. Everything was going smoothly right up until we were descending back into ZZZ. I am very sure none of the actions taken by myself of the crew had some sort of cause to this event but in case there was some sort of error I would like to find out what I need to be doing differently. As mentioned in an earlier section; I am not sure what would have caused this to happen. I would like to know what is discovered from this event and if there was something being done wrong; I want to learn from this.