Marcushope2

Hi all, last year I bought a 2012 Octavia RS (about 72,000km) and 2 weeks after I had a reading of a cylinder misfire. Took it to the local mechanic and he replaced the sparks but not the coils as he said he didn't have any on him (neither of which had ever been replace before). Next week, same issue, I took it back and he replaced the coils. Since then done about 12,000km and no worries. Today I get the same message, cylinder 3 misfire. I have swapped a couple of coils around to see if they were the issue, and it still came up with cylinder 3 misfire. I noticed on the top housing of one of the coils (cylinder 2) there was a little bit of oil which I thought was strange. My question are: Is this a common problem? If it is the spark plugs is that a job that's doable from home? Are the insides of the cylinder where the coils go meant to be squeaky clean? there was a few marks of something in there. Thanks for any help!!