2006 Nissan Frontier XE (2 wd) with SES codes P0420 and P0303

Hi!

Awhile back my truck started reading code P0420. I read that it was potentially related to the catalytic converter (as well as other things). It was running fine and folks told me it was just an emissions thing and I could wait. Approx a month ago- my truck started to make sort of a rattling sound when accelerating. this past Monday, my truck began to struggle. It's a manual transmission and every gear I was in felt as though I shifted too soon. It wasn't getting enough power, felt like maybe gears were sloping and it was going to die. Then I pulled code P0303 (cylinder 3 misfire) which makes total sense.

My father works on my vehicles. Based on the symptoms, I'm curious which order of operations for finding the issue folks would recommend? Replacing engine coil 3? Replacing spark plugs? Replacing cat 1? Both of these codes are pretty vague.