If its just 3 keys it’s probably due to dirty contacts. If the 3 keys are near each other, then that would also make me think dirty switches / contacts.
I bet if you took the keyboard apart you would see several rubber button type switches under the keys, probably 2 per key. These rubber buttons are probably all on one big sheet of rubber, which can be lifted off the circuit board easily. The circuit board under the rubber buttons will have a bunch of contacts, one per button. Clean the contacts with a bit of rubbing alcohol and some cloth and clean the underside of the rubber buttons. That should fix your problem.
If you find torn rubber, broken plastic on the keys or real switches instead of rubber buttons then you’re going to need to order parts from Yamaha.
This is probably a job that the average guy can do himself. Probably does not involve any soldering or electronic detective work. Just need be really careful when working inside and remember electronics are delicate, work slowly and carefully. Also, avoid static electricity, work with a bare foot touching the ground if you don’t have a good means of grounding yourself.
But yeah, a good electronic repair shop should be able to fix it. If the keys are weighted like a piano, I might not be comfortable sending it to Joe Blow repair, but I don’t think the keyboard you’re talking about is weighted.
