Buick regal 2001 LS Took in and Napa said third cyder was bad. Can i change just that spark plug?

I know nothing about cars and this is my sons car he bought for 500$ with 178000 miles. runs great overall but just the check engine light blinks sometimes and runs rough. Kinda smells like rotten eggs for a few minutes but then the light stops and starts to run fine. Thanks