I recently replaced a hallway ceiling light with a pendant, only to find that the supplied canopy was smaller (5") than the hole previously cut in the ceiling (6"). I'm hoping someone has encountered a similar problem and has devised an effective solution. (Attempting to repair the plasterboard doesn't seem like it would work.)
What you need is a ceiling medallion. That's a fancy name for "a circular thing you stick on the ceiling to look nice and cover a hole". A lot of them are designed to actually have a lamp hang from them, but there are some with a big hole in the middle which is exactly what you need here. There are simple ones:
and fancier ones:
Some are white or metallic and ready to use as-is. Some are designed to be painted. You can get them online but a local store where you can really see what they look like may be better.
If you don't think that a plain color medallion will look good, I would consider a thin (~1/8 - ~3/16) stainless steel disk about 7" inches in diameter (not too expensive on ebay). Cut three holes to match. Polish with brasso and/or use an appropriate grit sandpaper to match the finish on your canopy.
Hopefully, the holes can be fairly small. If you need to cut a large hole (>1/2"), you should consider something else. Cutting a large hole in stainless steel will be too difficult, a typical DIY person won't have the right tools.