To be honest I would try swapping the disc. I'm sure Capcom will let you send it in for a replacement if you contact them. If the only title you have ever had trouble with is MH3 it is very likely you got a messed up disc. It is rare, but not unheard of to have a few duds in a major disc pressing. I got a flubbed copy of NiGHTS that I had to have swapped a few years back. That said it never hurts to blow a little compressed air into the back of the Wii and clear out any dust bunnies. Though it doesn't sound like overheating is the problem. You could also use a CD or DVD cleaning kit, though I'm not sure if the Wii will play them if they arn't specifically for it. Contact Capcom to get a replacement disc for sure though.
Best of luck!
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