You of course have a problem of some sort. I would do the nut loosening sequence over again yourself. Three cylinders making no difference at all I would not think the engine could continue to run.

I could see three stronger cylinders and three just less so but still a noticeable drop. Do you know what number those three cylinders showing some issue are? For example cylinders number 4, 5, and 6.

Also make sure to do this after adding a fuel celane booster and getting it mixed in the fuel as Mr Hunter mentioned. An engine with no power contribution from three cylinders mind not being able to run normally except perhaps like a cement mixer should not start easily either.

The chances of three cylinders going bad all of a sudden compression wise while not impossible is not that great.